The API has been changed to take an explicit length for the number of
texture coordinates passed, and it's now documented that if there are
more layers to the current material than the number of texture coords
passed, then default coordinates will be generated for the other
layers.
cogl_material_rectangle should now handle the case where a single
sliced texture is supplied as a material layer by falling back to
cogl_texture_rectangle. We are nearly at the point that
cogl_texture_rectangle could be deprecated. A few issues remain
though, such as not considering waste in cogl_material_rectangle.
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