Draft: virgl: add support for DRI3 with vtest backend
Add support for resource export when using virgl vtest backend. That also allows us to tentatively use DRI3 to reduce number of copies when presenting. The LIBGL_ALWAYS_SOFTWARE
environment variable shouldn't be set to use that path.
I'm opening as a draft as there are probably a lot of corner cases not handled, but I want to get some feedback about the general idea.