Let the compositor size space filling panels

There were two issues preventing to let the compositor size such
panels:
- the window needs to be anchored on opposite edges, so LayerShellQt
  requests size 0 (meaning the compositor should deciude) in that dimension
- QtWayland currently will not size windows smaller than the requested
  minimum size. We can work around this by having a minimum size that's
  actually smaller than the screen dimension.
This fixes a horizontal and a vertical panel next to each other to no
longer overlap.
CCBUG:440571