Optimize image drawing

In various situations, the whole image is redrawn in RsttoImageViewer, which should be avoidable, for example on size and position criteria.

[Worked around in 6d20e746] One such situation is when the main window loses or regains focus. This problem was originally reported in #63 (closed) as a regression, but the fix was correct: it is the redraw decision that is the problem.

[Fixed in f240c4af] Another perhaps more problematic situation is when hovering over toolbar items: the image is then redrawn multiple times, which can make the toolbar difficult to use (even if button tooltips are disabled in the code). Maybe related to #51 (closed).

[Not sure it's avoidable] A last situation (but the list is certainly not exhaustive) is when minimizing and then re-displaying the main window.

This problem only has visible effects on large enough images, but then it can severely degrade the user experience.