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 7 minutes and 34 seconds (queued for 1 second)