Xi: return AlreadyGrabbed for key grabs > 255

Merged Peter Hutterer requested to merge whot/xserver:wip/prevent-high-keycode-grabs into master

We can't have high keycodes because everything in XKB relies on 8 bits. XI2's API allows for 32-bit keycodes so we have to take those but nothing in the server is really ready for this. The effect of this right now is that any high keycode grab is clipped to 255 and thus ends up grabbing a different key instead.

https://bugzilla.redhat.com/show_bug.cgi?id=1697804

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