vrend: relax fatal error for copy-blits from old guest drivers

Copy-blits between externally-allocated (GBM) surface resources that use
glCopyImageSubData trigger a fatal GL error since strict error checking
was enabled in commit 8d452da8. The
issue has since been fixed in mesa 20.x, but a short-term relaxation is
needed to restore functionality to old guests until a guest driver uprev
can be carried out.

Signed-off-by: Ryan Neph <ryanneph@google.com>
Part-of: <!808>
17 jobs for !808 with fix-copy-blits in 16 minutes and 28 seconds (queued for 2 seconds)
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