Uses WebPToSDL() generic function defined in vwebp_sdl.[ch].
This function is not included in the libextras library, because it
would bring in an SDL dependency. Probably too heavy for now.
WebPToSDL() is separate, because it will be called used by the javascript
version of libwebp (through emscripten build rules)
Change-Id: Ic85b36f8ce4784f46023656278f6480be6802834
