we use a static guard to only call SDL_Init() once.
Another option would be to call SDL_Quit(), but it doesn't seem to be
doing anything.
Also, fix the HTML code and add 'use strict' directive.
BUG=webp:354
Change-Id: I3c6421e2c1c8cc200556cd4092a0ead9a8b054ef
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
