WIP: RFC: vtest ARB_buffer_storage
Support using DRM dumb buffers as the backing store for persistent/coherent buffers when using glBufferStorage. vtest shares the prime buffer fd with the guest which can map it to gain access to the buffer contents.
This MR is built on top of patches cherry-picked from the main ARB_buffer_storage WIP proposal at: !374 (merged)
The corresponding WIP Mesa branch is at: https://gitlab.freedesktop.org/afrantzis/mesa/-/tree/vtest-buffer-storage