When rendering multi-view in side-by-side or top-bottom mode, we squash
the UI to half of its size and draw it twice on screen. That means the
cursor coordinates used for UI interaction don't match what's visible on
screen.
This commit is a little event system hack (tm) to fix this. It has some
small glitches with cursor grabbing, but nothing to bad.
We'll also use it for viewport HMD support.
D1350, thanks for the feedback @dfelinto!
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