clutter-offscreen-effect: Don't recreate the material when FBO changes

clutter-offscreen-effect: Don't recreate the material when FBO changes

Previously whenever the size of the FBO changes it would create a new
material and attach the texture to it. This is not good for Cogl
because it throws away any cached state for the material. In
test-rotate the size of the FBO changes constantly so it effectively
uses a new material every paint. For shader effects this also ends up
relinking the shader every paint because the linked programs are part
of the material state.