Merge branch 'blender-v2.91-release'

UI: Datatransfer modifier: set mix factor inactive when not in use

For Customdata layer copying, interpolation with the mixfactor is only
done for certain mix modes, now set the UI inactive if the mixfactor is
not in use.

Namely, the modes are the "Above / Below Threshold" which are only used
for flags, colors and normals and mixing is not supported in these cases.

Spotted while looking into T81914.

Differential Revision: https://developer.blender.org/D9327

Merge branch 'blender-v2.91-release'

Fix T82079: Missing viewport redraw changing volume slicing axis

Send appropriate notifiers (via rna_Volume_update_display).

Maniphest Tasks: T82079

Differential Revision: https://developer.blender.org/D9351

Merge branch 'blender-v2.91-release'

Merge branch 'blender-v2.91-release'

Fix T81934: Painting/Sculpting in ortho fails to let strokes pass through clipped geometry

Caused by rB7878adf49cff.

When getting the stroke location via raycast in ortho view, the above
commit flipped the condition of the check to perform adjustments on the
rays start/end. This would thus happen (even though it shouldnt),
resulting in wrong depth and stroke location.

Now just flip the condition back, so adjustments only happen when
clipping is OFF.

Maniphest Tasks: T81934

Differential Revision: https://developer.blender.org/D9318

Merge branch 'blender-v2.91-release'

Fix T81953: Python error in UV Editor Overlay popup

Leftover from rBe05ce1ea2029, 'use_image_editor_legacy_drawing' was
removed.

Maniphest Tasks: T81953

Differential Revision: https://developer.blender.org/D9310

Merge branch 'blender-v2.91-release'