Changes

NEWT Screen: Add virtual top-left origin getX()/getY() ; WindowsWindow.c: allow negative coordinates

- ScreenImpl:
  - Use Point & Dimension for holding virtual origin and size
  - updateScreenSize() -> updateVirtualScreenOriginAndSize()
  - DimensionImmutable getNativeScreenSizeImpl()
    -> void getVirtualScreenOriginAndSize(Point virtualOrigin, Dimension virtualSize)

- WindowImpl setFullscreen(true): Use Screen virtual origin

- WindowsWindow.c
  - For x/y coords use GET_X_LPARAM/GET_Y_LPARAM which casts '(int)(short)'
    to preserve negative coordinates.

  - NewtWindow_setVisiblePosSize() allow negative coordinates

NEWT Windows/X11: Remove missed negative coordinate restrictions.