C API: add sk_xfermode.h, impl, test

tests/CTest.cpp

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "sk_canvas.h"
 #include "sk_paint.h"
 #include "sk_surface.h"
@@ skipping 14 matching lines @@
     sk_paint_t* paint = sk_paint_new();
 
     sk_canvas_t* canvas = sk_surface_get_canvas(surface);
     sk_canvas_draw_paint(canvas, paint);
     REPORTER_ASSERT(reporter, 0xFF000000 == pixel[0]);
 
     sk_paint_set_color(paint, sk_color_set_argb(0xFF, 0xFF, 0xFF, 0xFF));
     sk_canvas_draw_paint(canvas, paint);
     REPORTER_ASSERT(reporter, 0xFFFFFFFF == pixel[0]);
 
+    sk_paint_set_xfermode_mode(paint, SRC_SK_XFERMODE_MODE);
+    sk_paint_set_color(paint, sk_color_set_argb(0x80, 0x80, 0x80, 0x80));
+    sk_canvas_draw_paint(canvas, paint);
+    REPORTER_ASSERT(reporter, 0x80404040 == pixel[0]);
+
     sk_paint_delete(paint);
     sk_surface_unref(surface);
 }
 
 DEF_TEST(C_API, reporter) {
     test_c(reporter);
 }