linux_dmabuf_v1: allow callbacks for checking dmabuf import

This allows compositors like gamescope to supply a callback allowing them to process dmabuf import checking. If the compositor doesn't use wlroots rendering it may wish to validate dmabufs itself with whatever logic it uses. This is opt-in, so no change in functionality unless a callback is provided.

Useful for things like !4055 which broaden the set of dmabufs wlroots might accept on multigpu systems.

I'll add this is my fist stab at this and I'm certainly not coming from the perspective of the potential users of the callback. I kept it pretty general but I'm open to any suggestions if there is a different way or different callback parameters that things like gamescope would like to see instead.