dmabuf refactoring

This is a bunch of work refactoring how dmabufs work.

For a start, I finally decided to make the big tables - the one for all the memory formats in gdkmemoryformat.c and the one for all the drm formats in gdkdmabuf.c - contain all the information necessary for those formats.

Then, I moved shuffled code around to finally make GdkDmabufDownloader into an interface and implemented that interface by GskGLRenderer. This is in preparation for the GPU renderers, which would not work with the current approach.