hw/xwin: Improve data returned for RANDR queries

Set a linear gamma ramp.  This avoids the xrandr command always warning
'Failed to get size of gamma for output default'
(perhaps we should be using GDI GetDeviceGammaRamp(), if possible?)

Make CRTC report non-zero physical dimensions initially
4 jobs for xwin-randr-fix-2 in 5 minutes and 57 seconds (queued for 1 second)
Status Job ID Name Coverage
  Docker Image
passed #368842
debian-testing

00:00:21

 
  Build And Test
passed #368843
autotools-build-and-test

00:05:36

passed #368844
meson-build-and-test

00:01:24

 
  External
passed continuous-integration/appveyor/branch