When the `WLR_GBM_EXPLICIT_UPGRADE` flag is set to 1, the GBM allocator
will attempt to "upgrade" buffers allocated with implicit modifiers to
be managed with explicit modifiers.
This is useful to enable the conservative modifier fallback for the
Vulkan renderer on drivers/hardware that fully support implicit and
explicit modifiers, including being able to report the modifier of an
implicitly allocated BO.
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.
Since we only use the backend capabilities here we can simply pass
them in directly. This allows other locations to create an allocator
even if they don't have a backend. They can simply specify the caps
they want instead.
