MetaShadow, MetaShapedTexture and MetaBackgroundActor all loop over sets of rectangles and draw multiple rectangles from the same texture with repeated cogl_rectangle_with_texture() calls.

We might get some speedup by using cogl_rectangles_with_texture_coordinates()

 - We'd have to manage a local rectangle array
 - But we save some per-rectangle material prep/validation

It's probably a loss as soon as you have to malloc the arrays, but if we can avoid that might be a small win.