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

include/c/sk_surface.h

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 // EXPERIMENTAL EXPERIMENTAL EXPERIMENTAL EXPERIMENTAL
 // DO NOT USE -- FOR INTERNAL TESTING ONLY
 
 #ifndef sk_surface_DEFINED
 #define sk_surface_DEFINED
 
 #include "sk_types.h"
 
 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
 
 #endif