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.

Action Constraint: introduce a mix mode setting.

Currently the action channels are applied after the existing
transformation, as if the action controlled a child of the
bone. This is not very natural, but more importantly, the
transform tools are not designed to work conveniently with an
additional 'pseudo-child' transformation, resulting in effects
like an unexpected pivot location.

Implementing a Before mode that integrates the action channels
as if applied to a parent allows using the special transform
tool code intended for dealing with such constraints.

Note that in either mode, Action constraints should be added
in reverse order, putting a new constraint before the existing
ones that the Action was keyframed to work together.

In order to implement the option, extract a utility from
the Copy Transform constraint code for combining transforms
with special anti-shear scale handling that matches the
Aligned Inherit Scale mode.

The Before mode also requires switching the constraint to
the Local owner space, while the After mode can still use the
World space for efficiency as before. Since the constraint
doesn't have an Owner space option in the UI, this has to be
handled in an RNA setter.

For full backward compatibility, the original simple matrix
multiplication mode is preserved as the third option, but it
is not recommended due to creating shear.

Differential Revision: https://developer.blender.org/D6297

UI: support Copy To Selected and Alt-Click for PropertyGroup members.

Rigify uses a property group to contain options of its rigs, so
currently it is impossible to use Alt-Click or Copy To Selected
to change a setting for multiple rigs at the same time.

The main problem here is that there is no efficient way to find
which bone the property group belongs to. To maintain performance,
implement this by checking the active bone if it is known. Copy
Data Path and related features still don't work, as data path
calculation can't use context.

Differential Revision: https://developer.blender.org/D6264

Fix T71213: Mask or Mirror before Armature breaks weight paint.

This is a revert of a small fraction of commit rB5e332fd700
that introduced the issue according to bisect.

Doing a break here is wrong, because BKE_crazyspace_build_sculpt
assumes that processing stopped at the first deform modifier
without deformMatrices, and thus skips all modifiers until it
finds one like that. Thus this early loop exit makes the behavior
worse, as instead of skipping just Mask and Mirror, it skips all.

Fix T71213: Mask or Mirror before Armature breaks weight paint.

This is a revert of a small fraction of commit rB5e332fd700
that introduced the issue according to bisect.

Doing a break here is wrong, because BKE_crazyspace_build_sculpt
assumes that processing stopped at the first deform modifier
without deformMatrices, and thus skips all modifiers until it
finds one like that. Thus this early loop exit makes the behavior
worse, as instead of skipping just Mask and Mirror, it skips all.