C API: Add SK_API, also documentation of an example.

experimental/c-api-example/c.md

+Skia's Stable C API
+===================
+
+<div style="text-align:center">
+<strong>EXPERIMENTAL EXPERIMENTAL EXPERIMENTAL EXPERIMENTAL<br>
+DO NOT USE &mdash; FOR INTERNAL TESTING ONLY</strong>
+</div>
+
+Several issues hinder the development of a stable ABI (application
+binary interface) for Skia:
+
+1.  Skia's C++ API changes a lot from version to version.  Skia's two
+    largest clients, Android and Chrome, are kept up to date by the
+    Skia team, but that can not happen for every client.
+2.  Skia's headers will only match the compiled skia libraries if
+    configured identically.
+
+To mitigate these two issues, Skia is experimenting with the
+introduction of a C API.  This will change more slowly than the C++
+interface and, once API version 1.0.0 is announced,
+backwards-incompatable changes will be avoided whenever possible.
+
+Here is an example program that uses the C api.  To try it out, get the file
+[`skia-c-example.c`](./skia-c-example.c).
+
+<!--?prettify lang=c?-->
+
+    #include <stdio.h>
+
+    #include "sk_data.h"
+    #include "sk_image.h"
+    #include "sk_canvas.h"
+    #include "sk_surface.h"
+    #include "sk_paint.h"
+    #include "sk_path.h"
+
+    static sk_surface_t* make_surface(int32_t w, int32_t h) {
+        sk_imageinfo_t info;
+        info.width = w;
+        info.height = h;
+        info.colorType = sk_colortype_get_default_8888();
+        info.alphaType = PREMUL_SK_ALPHATYPE;
+        return sk_surface_new_raster(&info, NULL);
+    }
+
+    static void emit_png(const char* path, sk_surface_t* surface) {
+        sk_image_t* image = sk_surface_new_image_snapshot(surface);
+        sk_data_t* data = sk_image_encode(image);
+        sk_image_unref(image);
+        FILE* f = fopen(path, "wb");
+        fwrite(sk_data_get_data(data), sk_data_get_size(data), 1, f);
+        fclose(f);
+        sk_data_unref(data);
+    }
+
+    void draw(sk_canvas_t* canvas) {
+        sk_paint_t* fill = sk_paint_new();
+        sk_paint_set_color(fill, sk_color_set_argb(0xFF, 0x00, 0x00, 0xFF));
+        sk_canvas_draw_paint(canvas, fill);
+
+        sk_paint_set_color(fill, sk_color_set_argb(0xFF, 0x00, 0xFF, 0xFF));
+        sk_rect_t rect;
+        rect.left = 100.0f;
+        rect.top = 100.0f;
+        rect.right = 540.0f;
+        rect.bottom = 380.0f;
+        sk_canvas_draw_rect(canvas, &rect, fill);
+
+        sk_paint_t* stroke = sk_paint_new();
+        sk_paint_set_color(stroke, sk_color_set_argb(0xFF, 0xFF, 0x00, 0x00));
+        sk_paint_set_antialias(stroke, true);
+        sk_paint_set_stroke(stroke, true);
+        sk_paint_set_stroke_width(stroke, 5.0f);
+        sk_path_t* path = sk_path_new();
+
+        sk_path_move_to(path, 50.0f, 50.0f);
+        sk_path_line_to(path, 590.0f, 50.0f);
+        sk_path_cubic_to(path, -490.0f, 50.0f, 1130.0f, 430.0f, 50.0f, 430.0f);
+        sk_path_line_to(path, 590.0f, 430.0f);
+        sk_canvas_draw_path(canvas, path, stroke);
+
+        sk_paint_set_color(fill, sk_color_set_argb(0x80, 0x00, 0xFF, 0x00));
+        sk_rect_t rect2;
+        rect2.left = 120.0f;
+        rect2.top = 120.0f;
+        rect2.right = 520.0f;
+        rect2.bottom = 360.0f;
+        sk_canvas_draw_oval(canvas, &rect2, fill);
+
+        sk_path_delete(path);
+        sk_paint_delete(stroke);
+        sk_paint_delete(fill);
+    }
+
+    int main() {
+        sk_surface_t* surface = make_surface(640, 480);
+        sk_canvas_t* canvas = sk_surface_get_canvas(surface);
+        draw(canvas);
+        emit_png("skia-c-example.png", surface);
+        sk_surface_unref(surface);
+        return 0;
+    }
+
+<a href="https://fiddle.skia.org/c/6c6c01438d9c3d80e9c22e606359432e"><img src="https://fiddle.skia.org/i/6c6c01438d9c3d8
+0e9c22e606359432e_raster.png" alt=""></a>
+
+Gyp+Linux example
+-----------------
+
+The following proof-of-concept workflow currently works on Ubuntu 14.04:
+
+1.  Aquire Skia and install dependencies (you may have already done this):
+
+    <!--?prettify lang=sh?-->
+
+        git clone 'https://chromium.googlesource.com/chromium/tools/depot_tools.git'
+        export PATH="$PWD/depot_tools:$PATH"
+        git clone 'https://skia.googlesource.com/skia'
+        skia/tools/install_dependencies.sh
+        SKIA_DIR="$PWD/skia"
+
+2.  Compile Skia as a shared library:
+
+    <!--?prettify lang=sh?-->
+
+        GYP_DEFINES=skia_shared_lib=1 "$SKIA_DIR"/bin/sync-and-gyp
+        ninja -C "$SKIA_DIR/out/Release" skia_lib
+
+3.  Compile, link, and run the example program:
+
+    <!--?prettify lang=sh?-->
+
+        cd [Wherever you want the example]
+        cp "$SKIA_DIR/experimental/c-api-example/skia-c-example.c" .
+        cc -c -I "$SKIA_DIR/include/c" skia-c-example.c -o skia-c-example.o
+        cc skia-c-example.o -L "$SKIA_DIR/out/Release/lib" -lskia -o skia-c-example
+        LD_LIBRARY_PATH="$SKIA_DIR/out/Release/lib" ./skia-c-example
+        xdg-open skia-c-example.png
+
+Cmake+MacOS example
+-------------------
+
+The following proof-of-concept workflow currently works on MacOS
+
+1.  Aquire Skia and install dependencies (you may have already done this):
+
+    <!--?prettify lang=sh?-->
+
+        cd [Wherever you want skia src code]
+        git clone 'https://skia.googlesource.com/skia'
+        SKIA_DIR="$PWD/skia"
+
+2.  Compile Skia as a shared library:
+
+    <!--?prettify lang=sh?-->
+
+        cd [Wherever you want skia build files]
+        mkdir build_skia
+        cd build_skia
+        SKIA_BUILD="$PWD"
+        cmake "$SKIA_DIR/cmake" -G Ninja && ninja
+
+3.  Compile, link, and run the example program:
+
+    <!--?prettify lang=sh?-->
+
+        cd [Wherever you want the example]
+        cp "$SKIA_DIR/experimental/c-api-example/skia-c-example.c" .
+        cc -c -I "$SKIA_DIR/include/c" skia-c-example.c -o skia-c-example.o
+        c++ skia-c-example.o \
+            "$SKIA_BUILD"/libskia.* -Wl,-rpath -Wl,"$SKIA_BUILD" \
+            -o skia-c-example
+        ./skia-c-example
+        open skia-c-example.png