xwayland: Expand the RANDR screen size limits

There's not really a good way to query this from the wayland server, so
just set the maximum to the X11 protocol limits. While we're at it,
lower the minimum screen size to something implausibly small too, just
in case.

Fixes: xorg/xserver#850
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
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