Regarding the GUI part, I had tested Jabier's branch, and one thing that hindered the drawing process was heavy onscreen display while rotating.
In my opinion there is no need to show the circular disc and clipping the canvas into that disc while rotating. a simple on-screen notification showing the degree of rotation is enough.
May be a small rectangle notification box with degrees. This is just my opinion. The previous rotation HUD kind of display was a bit distracting.
+1 for GUI notifications for rotation but with subtlety.
Thank you
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Raghavendra Kamath
Illustrator
raghukamath.com