dri2_glx: Put the invalidate b/c code back in

dri2_glx: Put the invalidate b/c code back in

The backwards compatibility code calls the DRI driver invalidate hook
on swap buffer and flush front buffer.  This lets the DRI driver rely
on invalidate callbacks and drop the glViewport() hack, even if the
server doesn't send invalidate events.  This is essentially a revert
of 2d00d16da7f5d2255cb37b48edaf4cbb9ca7e930, except that we now also
pass the __DRI_USE_INVALIDATE extension even when the server doesn't
have DRI2 invalidate events.