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

tests/CTest.cpp

Index: tests/CTest.cpp
diff --git a/tests/CTest.cpp b/tests/CTest.cpp
index f6ff4f0551a9f020ec1f492ddd54f0d65c66a113..1f301efa7e32474ee12836a230645c4f01f393b0 100644
--- a/tests/CTest.cpp
+++ b/tests/CTest.cpp
@@ -5,14 +5,13 @@
  * found in the LICENSE file.
  */
 
+#include "Test.h"
+
 #include "sk_canvas.h"
 #include "sk_paint.h"
 #include "sk_surface.h"
 #include "sk_shader.h"
 
-#include "Test.h"
-
-
 static void shader_test(skiatest::Reporter* reporter) {
     sk_imageinfo_t info =
         {64, 64, sk_colortype_get_default_8888(), PREMUL_SK_ALPHATYPE};