C API: add sk_xfermode.h, impl, test

tests/CTest.cpp

Index: tests/CTest.cpp
diff --git a/tests/CTest.cpp b/tests/CTest.cpp
index 413f629ee3f2f4bfc4f1f24a2b01e4a5817719c3..acab4bb11bfed6064bb1dbfb1dea7d5bab0e7c36 100644
--- a/tests/CTest.cpp
+++ b/tests/CTest.cpp
@@ -32,6 +32,11 @@ static void test_c(skiatest::Reporter* reporter) {
     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);
 }