Replace window repaints with layer repaints

Windows have two kinds of repaints - window repaints and layer repaints.
The main difference between the two is that the former is specified in
the window-local coordinates while the latter is specified in the global
screen coordinates.

Window repaints are useful in case the position of the window doesn't
matter, for example for repainting damaged regions, etc.

But its biggest issue is that with per screen rendering, it's not
possible to determine what screens exactly have to be repainted. The
final area affected by the window repaint will be known only at
compositing time. If a window gets damaged, we have to schedule a
repaint on ALL outputs. Understandably, this costs a little bit in terms
of performance.

This change replaces the window repaints with the layer repaints. By
doing so, we can avoid scheduling repaints on outputs that don't
intersect with the dirty region and improve performance.