Refactor texture offset snapping in TexturingViewOffsetTool

Signed-off-by: Kristian Duske <kristian.duske@gmail.com>
Merge scaling origin and rotation center into one concept. Fix a bug in TexturingViewRotationTool where the offset was wrong if the scale was not 1.

Signed-off-by: Kristian Duske <kristian.duske@gmail.com>