[mutter-shaped-texture] Use the new material api instead of raw GL calls
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.
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