[Piglit] [PATCH] glx: Fix make-current

[Ernestas Kulik]

Actually, scratch that, I must have been high when I wrote the commit
message.

Even though the fix for BadMatch is probably correct, glClear() is
obviously called for both drawables, but switching them between drawing
and reading seems to result in discarded framebuffers or somesuch (in
i965 they are straight up null, unlike swrast, but the issue manifests
there as well).