Blender Git Loki

Fix T43989: Sequencer - Ctrl snapping a sequencer strip does not work if you specify the x axis.

New 'strip' snapping was simply not computed in case of constrained transform, hence init
'0' value was used as frame offset in this case.

This commit reorganizes a bit that snapping, to keep it more 'confined' into `snapSequenceBounds()`
dedicated function. It still needs a minor hack (setting snapping mode to something else than
defualt `SCE_SNAP_MODE_INCREMENT`, to avoid this snapping to be called by contraint code).

Thanks to Antony for review and enhancements.

This fix should be backported to 2.74.