Add PNG support to SDL surfaces

[DaVince]

Self-explanatory. If not too much effort, the PNG library could be tied in with the SDL surfaces to make said surface support more useful.

[tung]

Looks doable if I implement SDL_CreateRGBSurfaceFrom. Despite the name, the man page says that it supports the alpha channel too.

[tung]

Add CreateRGBSurfaceFrom to SDL lib. Closed by 73c60ba.

• See test/blitpng.lua for an example of how to use it.

[DaVince]

Works great, even when I change the background of the smiley to transparency. Two blitted smileys will overlap properly, keeping the transparency in a ccount.