GLES: u_TextureMatrix with 2 vec4s instead of 3 vec2s

One uniform compression taken from the GLES version.

u_TextureMatrix[0] and u_TextureMatrix[1] are now packed in a single vec4. This may save one uniform vector.

I don't want to use `#ifdef USE_GLES2` in this piece of code because the change can be easily applied to the desktop renderer code.