Previously picking was done on an int (x,y) to address a particular pixel.
While `int` was the minimum precision required, it was also an unnecessary
type conversion.
The callers (input events mainly) all provide float coordinates and the
internal picking calculations also have always used floats. So it was
inconsistent and unnecessary to drop to integer precision in between those.
ABI break: This changes the parameter types for public function
`clutter_stage_get_actor_at_pos`, but its documentation is already
sufficiently vague to not need changing.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/844
8c89ea5f0a