Fixes and updates for Ginga viewer

	modified:   BaseImage.py
	modified:   FitsImage.py
	modified:   ginga.py
	modified:   gtkw/plugins/Zoom.py
	modified:   qtw/plugins/Zoom.py

- added a sanity check on scaling
  - prevents large unecessary arrays from being generated and slowness
  - limit is approx when one pixel fills the window
- changed zoom plugin to use the "mark pan position" feature instead of
    drawing a small rectangle
  - this speeds things up a little and looks better