Changes

Nativewindow Rectangle*: Cleanup up union(..), intersection(..): Return a new Rectangle instance (mutable)

Also return 'this' for setter methods for chaining.

Bug 1422: Emulate DPI Scaling on non-native DPI autoscale platforms (!MacOS)

Bug 1422 shows that it seems to be desired to emulate DPI scaling where
the native toolkit does not implmement the same.

On GTK, DPIUtil.mapDPIToZoom (int dpi) reads:
  double zoom = (double) dpi * 100 / 96;
  int roundedZoom = (int) Math.round (zoom);
  return roundedZoom;

While having dpi calculated as:

  dpi = 96 * GDK.gdk_monitor_get_scale_factor(monitor);

Well, this seems to exist to allow 96 dpi fixed layout to
'look' OK on high-dpi screens.
However, you get in trouble if you layout high-dpi aware,
i.e. using percentages etc.

There is one exception: If DPIUtil.useCairoAutoScale() is true, scalingFactor is 1f
and hence the scaling emulation dropped.

'DPIUtil.setUseCairoAutoScale((sx[0]*100) == scaleFactor || OS.isGNOME);'