backends/virtual: Port to gbm

The virtual backend uses the surfaceless platform. On the other hand, we
move in a direction where the graphics buffer type is explicit, which
creates issues for the virtual backend.

This change ports the virtual backend to gbm so we could manually
allocate dmabuf buffers in order to unify buffer handling in kwin.

Its main drawback is that you won't be able to use the virtual backend
on setups without render nodes. On the other hand, given that the
compositor is meaningless without clients being able to share buffers
with it, it's reasonable to require some way to create and export prime
buffers.