Edge Slide: use snap system to check sliding vertex visibility
This is a different solution to #32068
The sliding edge occlusion test is done to prevent a non-visible
reference vertex from being used to calculate the factor direction.
This commit changes the behavior, but the difference is difficult to
notice.
The changes in behavior are:
- Other objects affect occlusion.
- Slide direction is used to test occlusion instead of edge.
- The reference point is now the point whose direction is visible and
is closest to the mouse cursor.