xephyr: Don't forget to glViewport() before drawing the screen.

xephyr: Don't forget to glViewport() before drawing the screen.

Fixes misrendering with cairogears.  I had noticed the failure while
trying to figure out what was going on with traps.  Cairogears was
apparently putting its results on the screen through putimage, which
is a texture upload, so the last GL drawing was done to the size of
the cairogears window, not the size of the xephyr screen.

Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Keith Packard <keithp@keithp.com>