[Fluxus] multitexture automatic checking

[gabor papp]

> i added the missing include file to the cpp files. now it compiles, but
> crashes on my macbook:
i committed a fix that checks for the availability of the
glActiveTexture function, because sometimes NULL pointer is returned.

it would be nice to display a message if multitexturing is not
supported. i tried to put it in texturepainter initialise, but it is
called every frame, so probably that's not the best place for this.

gabor