Correctly draw window snapping hints on multi-head systems
Previously, the transparent frames that were drawn prior to snapping a
window assumed that there was only one head, i.e., that the new
position and dimensions of the window would be based on the dimensions
of the entire screen.
However, this is not the case on multi-head systems, and so we now
base the transparent frame's position and dimensions on the current
head of the window.
We also refactor the code so that the new dimensions are computed in
the switch statement and finish with one final call to
drawTransparentFrame.