Keep track of whether "Zoom to Window" is active
Keep track of whether "Zoom to Window" is active

- if the user selects "Zoom to Window", but then manually changes the
zoom level, we exit "Zoom to Window" mode

- if we are in "Zoom to Window" mode, and UpdateCanvasScale is called
without the user manually zooming (e.g. after rotating or resizing image),
we need to stay in "Zoom to Window" mode and recompute the correct
zoom level --> this requires that we keep track of the "Zoom to Window"
state as well as the previous zoom level

jakeclawson/Pinta,jakeclawson/Pinta,PintaProject/Pinta,Fenex/Pinta,Fenex/Pinta,Mailaender/Pinta,PintaProject/Pinta,PintaProject/Pinta,Mailaender/Pinta