Code review comment for lp:~mir-team/mir/custom-input-regions-scale-with-surface-size

Come to think of it, the solution is simple:

1. Client-specified region is always intersected with the surface region to get the final result.
2. The default (when client hasn't specified) input region is an "infinite region". But intersected with the surface region that gives you just the surface.
3. Right answer every time.