[Fluxus] multitexture automatic checking

[gabor papp]

> Thanks for that Gabor - do I take it that GLSL isn't really a problem,
> so we should have this fixed now?
it works for me now, but it would be nice to get more feedback from
others who were experiencing this problem before. what is strange that
glew sometimes returns the function pointer, sometimes not depending on
the build.

it would be nice to autodetect glsl support also, so we could get rid of
the compile parameters completely.

> I've added a warning in the TexturePainter constructor.
great. thanks.

gabor