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

include/c/sk_image.h

Index: include/c/sk_image.h
diff --git a/include/c/sk_image.h b/include/c/sk_image.h
index c9973ac1252a916a96b6a129e4219ff4e8559eb6..e6f49b74ee89b1819f7afa1ad725822165e15999 100644
--- a/include/c/sk_image.h
+++ b/include/c/sk_image.h
@@ -19,7 +19,7 @@ SK_C_PLUS_PLUS_BEGIN_GUARD
  *  Return a new image that has made a copy of the provided pixels, or NULL on failure.
  *  Balance with a call to sk_image_unref().
  */
-sk_image_t* sk_image_new_raster_copy(const sk_imageinfo_t*, const void* pixels, size_t rowBytes);
+SK_API sk_image_t* sk_image_new_raster_copy(const sk_imageinfo_t*, const void* pixels, size_t rowBytes);
 
 /**
  *  If the specified data can be interpreted as a compressed image (e.g. PNG or JPEG) then this
@@ -28,15 +28,15 @@ sk_image_t* sk_image_new_raster_copy(const sk_imageinfo_t*, const void* pixels,
  *  On success, the encoded data may be processed immediately, or it may be ref()'d for later
  *  use.
  */
-sk_image_t* sk_image_new_from_encoded(const sk_data_t* encoded, const sk_irect_t* subset);
+SK_API sk_image_t* sk_image_new_from_encoded(const sk_data_t* encoded, const sk_irect_t* subset);
 
-sk_data_t* sk_image_encode(const sk_image_t*);
+SK_API sk_data_t* sk_image_encode(const sk_image_t*);
 
-void sk_image_ref(const sk_image_t*);
-void sk_image_unref(const sk_image_t*);
-int sk_image_get_width(const sk_image_t*);
-int sk_image_get_height(const sk_image_t*);
-uint32_t sk_image_get_unique_id(const sk_image_t*);
+SK_API void sk_image_ref(const sk_image_t*);
+SK_API void sk_image_unref(const sk_image_t*);
+SK_API int sk_image_get_width(const sk_image_t*);
+SK_API int sk_image_get_height(const sk_image_t*);
+SK_API uint32_t sk_image_get_unique_id(const sk_image_t*);
 
 SK_C_PLUS_PLUS_END_GUARD