Philip Langdale authored Nov 19, 2018 and Niklas Haas committed Nov 21, 2018

When we introduce exportable semaphores, we'll have handles which are
not memory-backed. To avoid a sloppy API, let's explicitly separate the
two ideas. This also presents an opportunity to pull the object offset
within the memory into the mem_handle definition. We also refactor the
way the handle caps are presented to the user, to allow distinguishing
between caps for shared memory and those for semaphores.

We also turn the struct pl_gpu_handle into a union, since in practice,
it is not going to be useful to export a given resource as multiple
types of external handle - it will just one or another type. (This is
mostly a simple substitution, but we must also note that vk_slabs must
also carry the handle type explicitly, as we can no longer look at which
field(s) are set in a pl_gpu_handle.)

This is obviously an API break for any external usage of buffers or
textures.