udmabuf can create a DMA-BUF backed by a memfd. This is useful
when running with a software implementation of GL/Vulkan: the memfd
can be passed to the parent compositor via wl_shm and the DMA-BUF
can be imported via the usual APIs into GL/Vulkan.
Now that the DRM backend no longer depends on GBM, we can make it
optional. The GLES2 renderer still depends on it because of our EGL
device selection.
This is useful for compositors with their own renderers, and for
compositors using the Vulkan renderer.