Snap to exact 15 degree increments when using discrete rotate
I missed this behavior when working on !1779 (merged). Currently the discrete rotation will simply add a 15 degree increment to the canvas rotation. Instead, it should first snap to the nearest multiple of 15 degrees.
I have verified the behavior on Windows 11.
If accepted, this should be cherry-picked to krita/5.2 please.
Edited by Maciej Jesionowski