Now that Cogl has the material API we can use Cogl calls for
multi-texturing instead of directly calling GL.

The MutterShapedTexture actor keeps a handle to a material with the
right combine modes for combining the texture and the shape
mask. Before each paint it sets the layers of the material to be the
two textures.

A second material with different combine modes 1is also created if the
pixmap is 24 bit to implement the workaround from commit 72da8c57.