GdkDmabuf is a struct encapsulating all the values of a dmabuf, so
nothing to see here.
GdkDmabufDownloader is a vtable for a thing that can download dmabufs.
For now only one implementation exists, so this just looks like a ton
of work for no benefit.
The only neat thing is that gdkdmabuftexture.c got a whole lot tidier.
Add an implementation of GdkDmabufTexture.
For now, this implementation is rather minimal,
since we need a roundtrip through GL to convert
most nottrivial formats.
Add a builder for a new GdkTexture subclass that
wraps dmabuf buffers on Linux. For now, this is
just an API. The implementation will follow in
subsequent commits.
