Implement dmabufs in pixman renderer
In the pixman renderer, hold a DRM FD (if available) and add support for importing linux-dmabuf buffers. This allows hardware-accelerated GL applications to run even if the compositor is using pixman rendering, and in future may allow direct scanout of video under the pixman renderer.
The main limitation is that only single-plane formats are supported, so no support for YUV or NV12.
Closes #3844