For UGA, the frame buffer address obtained by scanning the
PCI BARs does not necessarily correspond to the upper-left
most pixel. Scan the frame buffer for which byte changed
when changing the pixel at (0,0).

Use the same technique to determine the stride. Except for
changing the pixel at (0,0), we change the pixel at (0,1).

PR: 202730
Tested by: hartzell (at) alerce.com