qxl: stride fixup

spice uses negative stride value to signal the bitmap is upside down.
The qxl renderer (used for scl, vnc and screenshots) wants a positive
value because it is easier to work with.  The positive value is then
stored in the very same variable, which has the drawback that the
upside-down test works only once.  Fix by using two variables.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>