More SDL2 fixes

From what I can see, the only remaining tasks include:
* Implement checkPixel() ~~and readPixel()~~. I don't know how to do this (yet) since it's near impossible to read SDL_Texture pixel data.
* ~~Implement scaleSurface(). This will probably be similar to the SDL1 version, so we'll need to have readPixel() implemented~~

Update: I found a way to implement scaleSurface() without needing readPixel(). So now all that's missing is a checkPixel() implementation.