Move graphics reset handling to RenderBackend
This makes the Scene less overloaded and it's needed for things such as render layers.
In hindsight, it would be great to merge checkGraphicsReset() and beginFrame(), e.g. make beginFrame() return the status like in QRhi or VkSwapchain. If it's OUT_OF_DATE or something, reinitialize the compositor.
Note it's not been tested with an NVIDIA video card.