Blender Git Loki

Fix T66802: Edge Slide: Unpredictable direction of the edge loop when it is fully occluded

When an edge loop is fully occluded the direction of movement
is not calculated resulting in unpredictable behavior.

So always calculate the direction of the groups of edge loops
but continue preventing occluded edges from affecting `mval_dir`
(global) value.

Differential Revision: https://developer.blender.org/D5247