The purpose of rrCheckPixmapBounding is to make sure that the
screen_pixmap is large enough for the slave-output which crtc is
being configured.

This should include crtc->x and crtc->y, otherwise the crtc might
still end up scanning out an area outside of the screen-pixmap.

For example: Take a laptop with an external monitor on a slave-output at
1920x1080+0+0 and its internal-screen at 3840x2160+1920+0 and in
gnome-settings-daemon move the external monitor to be on the ri ght of
the internal screen rather then on the left. First g-s-d will do a
RRSetScreenSize to 5760*2160 (which is a nop), then it calls RRSetCrtc
to move the slave output to 1920x1080+3840+0, since this is a slave
output, rrCheckPixmapBounding gets called, since the 2 crtcs now overlap
the code before this commit would shrinks the screen_pixmap to 3180*2160.
Then g-s-d calls RRSetCrtc to move the internal screen to 3180*2160+0+0.

And we end up with the slave-output configured to scan-out an area
which completely falls outside of the screen-pixmap (and end up with
a black display on the external monitor).

This commit fixes this by not substracting the x1 and y1 coordinates
of the union-ed region when determining the new screen_pixmap size.

Cc: Nikhil Mahale <nmahale@nvidia.com>
Cc: Dave Airlie <airlied@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>