Deps: Add PugiXML as an official dependency

PugiXML was historically shipped hidden embedded into OIIO, the Grease
Pencil team had a requirement for an XML library recently so pugi seems
like a natural choice since it's not really a 'new' library, we just
turn an implicit dependency into an explicit one.

This commit expands the Windows-specific code in rBdca9aa0053f7 to
include Linux. macOS support will be handled in a later commit.

NOTE: run `cmake -U'*PUGIXML*' .` in the build directory to ensure CMake
finds PugiXML in the new location.

For details see D8628

Cleanup: Animation, clean up action group rearranging code

Some minor cleanups to make an upcoming bugfix easier.

No functional changes.

Fix T81628: Moving Python-made channels freezes Blender

Fix various problems in the Action Group rearranging code. All fixes are
necessary to resolve the bug.

- Before groups are rearranged, the channels are moved into their
respective groups (so no longer referenced by `action->channels`). A
temporary group is made for ungrouped channels. The code made
assumptions about the channels being in the same order as the groups;
that assumption has been removed.
- Looping over channels in an Action Group should stop when reaching the
last channel, and not until `NULL`.
- After all the reshuffling is done, the `action->channels` linked list
wasn't terminated properly. Now `first.prev` and `last.next` are set
to `NULL` to avoid infinite loops.

Fix T82156: Object with constraints translates when parented

Avoid the evaluation of constraints when computing the parent-inverse
matrix.

Constraints are meant to be evaluated last; object transforms are
computed this order:

1. `parent->obmat` (the parent object's world matrix)
2. `ob->parentinv` (the object's parent-inverse matrix)
3. Object's loc/rot/scale
4. Object's constraint evaluation

When the constraints are used to compute the parent-inverse matrix,
their effect is moved from step 4 to step 2 in this list, potentially
rotating or scaling the object's local transform. This causes unwanted
movement as reported in T82156.

Reviewed By: looch

Differential Revision: https://developer.blender.org/D9413

Cleanup: Animation, clean up FCurve Cycles modifier

Simplify conditions and declare variables `const` where possible.

No functional changes.

Cleanup: clearer marking of what was changed in GTestAddTests.cmake

Replace `Blender` with `BLENDER` so that it's more clearly a marker, and
not just the name.

No functional changes.

Merge remote-tracking branch 'origin/blender-v2.91-release'

Graph Editor: fix keyframe not activating in certain cases

When clicking on an already-selected keyframe, mark it as active if the
click caused the previously-active keyframe to become deselected.

When clicking on a key in the graph editor, it is selected and all other
keys are deselected. If that key was already selected before the click,
it would not become the active keyframe. This is now fixed.

Reviewed by: Severin

Differential Revision: https://developer.blender.org/D9639

Animation: Remove debug prints

Remove accidentally-commited debug prints.

No functional changes.

Reviewed by: sergey

Differential Revision: https://developer.blender.org/D9637

Animation: New Euler filter implementation

This new discontinuity filter performs actions on the entire Euler
rotation, rather than only on the individual X/Y/Z channels. This makes
it fix a wider range of discontinuities, for example those in T52744.

The filter now runs twice on the selected channels, in this order:
- New: Convert X+Y+Z rotation to matrix, then back to Euler angles.
- Old: Add/remove factors of 360° to minimize jumps.

The messaging is streamlined; it now reports how many channels were
filtered, and only warns (instead of errors) when there was an actual
problem with the selected channels (like selecting three or more
channels, but without X/Y/Z triplet).

A new kernel function `BKE_fcurve_keyframe_move_value_with_handles()` is
introduced, to make it possible to move a keyframe's value and move its
handles at the same time.

Manifest Task: T52744

Reviewed By: looch

Differential Revision: https://developer.blender.org/D9602