Bug 741 HiDPI: Simplify ScalableSurface [set|get]SurfaceScale(..) spec, which also fixed JAWTWindow getSurfaceScale() issue on Windows

Let setSurfaceScale(..) return the validated requested values
and getSurfaceScale(..) always the current values.

This removes complication and solves a bug w/ JAWTWindow on Windows,
where we used 'drawable' as an indicator for 'previous locked' state.
The latter is not true since on Windows 'drawable' is set to null in unlock,
getWindowHandle() should be taken instead.