scenes/opengl: Use hardware clipping for clearing the background

Right now, when `paintBackground()` is called to clear the background,
we use custom geometry and render that when the damage region isn't
infinite. Rather than using geometry, we can instead use `glScissor()`
to set the area that needs to be cleared and then use `glClear()` to
clear it.

In addition, if we have only one rect in the damage region and that rect
matches the render target rect, we can completely skip all that setup
and use `glClear()` directly.