RandR 1.0 sizeID must be computed the same way every time, so when reporting
it in the ScreenChangeNotify event, just construct the usual 1.0 data block
and use that.
subpixel geometry information can be computed by looking at the connected
outputs and finding any with subpixel geometry and using one of those for
the global screen subpixel geometry. This might be improved by reporting
None if more than one screen has information and they conflict.
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