ShowBase: Minor fixes over aspect ratio and size calculations
ShowBase: Minor fixes over aspect ratio and size calculations

        - Do not take into accound SBS if it is not enabled
        - Always return value for getSize()
        - Properly handle SBS for window-events
        - Redo positioning of aspect2d(p) markers exactly as
        they are created when adjusting aspect ratio

Signed-off-by: deflected <4e8b39f4fad01cf7ba570fbff685e6b412438ff4@users.noreply.github.com>

chandler14362/panda3d,chandler14362/panda3d,chandler14362/panda3d,chandler14362/panda3d,chandler14362/panda3d,chandler14362/panda3d,chandler14362/panda3d,chandler14362/panda3d,chandler14362/panda3d,chandler14362/panda3d