Now all options for "snap to" affect the Vert Slide mode.
Reviewed By: campbellbarton
Maniphest Tasks: T66426
Differential Revision: https://developer.blender.org/D3440
Currently, this change does not bring functional changes.
But it is necessary to extend the use of the snap system for gizmos,
since, after a Undo, the `depsgraph` pointed by the `snap_context`
has its memory invalidated.
Reviewed By: campbellbarton
Differential Revision: https://developer.blender.org/D7013
