Unify beginFrame() and endFrame() hooks for OpenGL and QPainter backends

This unifies frame hooks for OpenGL and QPainter render backends. There are a couple of reasons why it's a good idea - it provides one mental framework to start painting a frame, the Compositor will be able to start and submit frames. The last one is very cool because it gives the Compositor more power over compositing.

Besides unifying frame hooks, this cleans up a bit the arg naming mess in endFrame(). As is, "damage" and "damagedRegion" are very confusing names. "damage" arg has been renamed to "renderedRegion," because that's what it is. The renderedRegion arg specifies the region that has been repainted by the Scene. It's different from the damagedRegion as that one specifies the surface damage, i.e. the difference between the current and the next frame, while the renderedRegion may include a region that had to be repainted to repair the back buffer. The main reason why we need renderedRegion is the X11 platform. On Wayland, it's unused.

In the future, we will need to extend this api with output layers.