gl-renderer: make DMABUF allocator optional

The dmabuf allocator pulls in libgbm which lives inside Mesa and is built
as a non-standalone part Mesa. This makes gl-renderer hard-wired to Mesa.

The allocator is only used by an optional pipewire backend so make
the allocator optional too.

Signed-off-by: Tomek Bury <tomek.bury@gmail.com>