Wayland compositor: Implement linux_dmabuf

linux_dmabuf is an unstable Wayland protocol extension. From "linux-dmabuf-unstable-v1.qml":

This interface offers ways to create generic dmabuf-based
wl_buffers. Immediately after a client binds to this interface,
the set of supported formats and format modifiers is sent with
'format' and 'modifier' events.

On the compositor side, the DMA buffers can for instance be used for hardware compositing (and avoid an unnecessary copy of the buffers).

This protocol is also required to support the etnaviv (mesa open-source drivers for vivante, used on iMX 6, see linked bug report)

May enable other performance improvements on various drivers that use buffer modifiers: https://www.collabora.com/news-and-blog/blog/2018/03/23/a-new-era-for-linux-low-level-graphics-part-2/

Entire protocol: https://github.com/wayland-project/wayland-protocols/blob/c438cbe7dc6d7356fd64d79e8e7159d025b16bbc/unstable/linux-dmabuf/linux-dmabuf-unstable-v1.xml