When painting using Auto-Normalize or Lock Relative with some
groups locked, the locked weights may not add up precisely to
1 because of precision limitations, which results in creating
nonzero weights close to FLT_EPSILON. With Lock Relative display
mode this is very obvious and annoying (random red points amid
black or blue), so add an epsilon check to consider less than
1e-6 unlocked weight to be the same as 0.
In addition, in cases when no weight can be painted due to locks,
don't create vertex group entries at all if they don't exist yet.
Also, don't run Auto Normalize when not painting a deform group.
Differential Revision: https://developer.blender.org/D10000
58dae919e5