Weight Paint: Implement a new Lock-Relative mode.

This check box alters how weights are displayed and painted,
similar to Multi Paint, but in a different way. Specifically,
weights are presented as if all locked vertex groups were
deleted, and the remaining deform groups normalized.

The new feature is intended for use when balancing weights within
a group of bones while all others are locked. Enabling the option
presents weight as if the locked bones didn't exist, and their
weight was proportionally redistributed to the editable bones.

Conversely, the Multi-Paint feature allows balancing a group of
bones as a whole against all unselected bones, while ignoring
weight distribution within the selected group.

This mode also allows temporarily viewing non-normalized weights
as if they were normalized, without actually changing the values.

Differential Revision: https://developer.blender.org/D3837

Weight Paint: add a pie menu for locking and unlocking vertex groups.

Provide different options for locking and unlocking vertex groups
using bone selection, accessible via a pie menu triggered via the
'K' hotkey. To implement a variety of operations, extend the old
operator with a new option to mask it by bone selection. If the
X Mirror option is enabled, selection is automatically mirrored.

This follows D6533 as the next step in improving accessibility of
vertex group locking during weight painting.

Differential Revision: https://developer.blender.org/D6618

Merge branch 'blender-v2.82-release'

Depsgraph: fix false positive time dependencies for simple drivers.

The dependency graph has to know whether a driver must be re-evaluated
every frame due to a dependency on the current frame number. For python
drivers it was using a heuristic based on searching for certain sub-
strings in the expression, notably including '('.

When the expression is actually evaluated using Python, this can't be
easily improved; however if the Simple Expression evaluator is used,
this check can be done precisely by accessing the parsed data.

Differential Revision: https://developer.blender.org/D6624

Merge branch 'blender-v2.82-release'

Fix T73117: B-Bone twist weirdness in chains with sharp bends.

When computing the roll value coming from the handle bone, the code
was using some strange unexplained math. It probably works fine when
the difference with the 'zero roll' orientation is pure twist, like
is the case when called from mat3_to_vec_roll. However, it appears
to break when significant swing is involved.

The issue is fixed by using the proper Swing+Twist decomposition
utility function that was added in a recent version for drivers.

Weight Paint: implement a red shade for bones with locked weights.

Blender supports locking vertex groups to prevent changes to the
weights. However, as mentioned in comments for D3837, it is hard
to use this because there is no interface for locking in 3D View.

This adds a red shade to bones that are associated with a locked
weight group during weight paint mode, as the first step to adding
such interface. The next step is adding a pie menu for lock/unlock.

Differential Revision: https://developer.blender.org/D6533

Fix T57159: don't exit tweak mode in Bake Action if reusing the action.

Fix T69105: reset the action blend mode to Replace in Bake Action.

This can't be done in Tweak mode since the setting is stored in the
tweaked strip, and the logic for finding it is not exposed to python.

Shrinkwrap: improve triangle boundary stability in Target Normal Project.

Rewrite the checks for determining if the solution is actually within
the triangle to fix stability issues when the correct solution is on
an edge, and step is very small, i.e. the solution is already very
close. Also, comment more clearly what is happening geometrically.

This should fix problems when vertices that should project exactly
onto an edge actually miss, resulting in weird spikes. This made
Target Normal Project unusable for the voxel remesher.