loader_dri3: Make sure we have an updated back v3

loader_dri3: Make sure we have an updated back v3

With GLX_SWAP_COPY_OML and GLX_SWAP_EXCHANGE_OML it may happen in situations
when glXSwapBuffers() is immediately followed by for example another
glXSwapBuffers() or glXCopyBuffers() or back buffer age querying, that we
haven't yet allocated and initialized a new back buffer because there was
no GL rendering in between.

Make sure that we have a back buffer in those situations.

v2: Eliminate the drawable have_back_format member.
v3: Make sure we re-initialize the back even if it exists.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>