DrmBuffers behave very differently if created from a GBM surface or as a dumb QImage buffer. Because of that split it up in subclasses to increase clarity and minimize conditions in the rendering loop. Also this will help, when we later introduce direct scanouts from wayland buffers without rendering on planes (just another subclass).
Also the buffer deletion was wrong in my last work on Atomic Mode Setting. QPainter buffers shouldn't be deleted between page flips, because they reuse the buffers.