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

include/c/sk_surface.h

Index: include/c/sk_surface.h
diff --git a/include/c/sk_surface.h b/include/c/sk_surface.h
index 397dc833c2b63b00a9eeeeba1ceae6c2115eb23c..f3c17d075d542244978b230181cfb665c0aa8721 100644
--- a/include/c/sk_surface.h
+++ b/include/c/sk_surface.h
@@ -15,21 +15,22 @@
 
 SK_C_PLUS_PLUS_BEGIN_GUARD
 
-sk_surface_t* sk_surface_new_raster(const sk_imageinfo_t*, const sk_surfaceprops_t*);
-sk_surface_t* sk_surface_new_raster_direct(const sk_imageinfo_t*, void* pixels, size_t rowBytes,
-                                           const sk_surfaceprops_t* props);
-void sk_surface_unref(sk_surface_t*);
+SK_API sk_surface_t* sk_surface_new_raster(const sk_imageinfo_t*, const sk_surfaceprops_t*);
+SK_API sk_surface_t* sk_surface_new_raster_direct(const sk_imageinfo_t*,
+                                                  void* pixels, size_t rowBytes,
+                                                  const sk_surfaceprops_t* props);
+SK_API void sk_surface_unref(sk_surface_t*);
 
 /**
  *  Return the canvas associated with this surface. Note: the canvas is owned by the surface,
  *  so the returned object is only valid while the owning surface is valid.
  */
-sk_canvas_t* sk_surface_get_canvas(sk_surface_t*);
+SK_API sk_canvas_t* sk_surface_get_canvas(sk_surface_t*);
 
 /**
  *  Call sk_image_unref() when the returned image is no longer used.
  */
-sk_image_t* sk_surface_new_image_snapshot(sk_surface_t*);
+SK_API sk_image_t* sk_surface_new_image_snapshot(sk_surface_t*);
 
 SK_C_PLUS_PLUS_END_GUARD