GraphicsContext::readFramebuffer calls Q_UNREACHABLE if an unsupported format is used

A texture may easily be set to one of the unsupported formats even though a RenderCapture is part of the frame graph. When GraphicsContext::readFramebuffer calls Q_UNREACHABLE, this leads to undefined behavior, which on my machine means the program just hangs in the render thread.

We should rather return an empty image and print a warning in these cases.