Separate alpha texture support

[atteneder]

Add ability to export color and alpha as separate textures.
Makes sense on platforms where none of the compressed alpha texture formats is supported.
Some shaders might also work like that , so it might be usefule anyways.

Use-case: No alpha texture on WebGL Android Chrome.

[kyapp69]

I also second this as on platforms which does not support alpha, we would create a specific shader that uses a greyscale texture as an alphamask.