Fix potential issues when loading files with missing libraries

This is a continuation of rB39f005eae8eed8b939579aff8c9a05a4f50e5e38

Now all the fields where we check for object type in RNA (like
rna_Curve_object_poll) will have a safe guard for when this isn't the
case. For example when loading files that has missing object libraries
and all missing objects are replaced with empties (placeholders).

Reviewed By: Brecht

Differential Revision: http://developer.blender.org/D5425

Update Eigen to 3.3.7

This is in preparation for the QuadriFlow remesher lib.

Reviewed By: Brecht

Differential Revision: http://developer.blender.org/D5549

Fix building with shared OIDN libraries

Previously cmake would error out if it couldn't find the static
libraries even though it can build with shared libraries just fine.

Fix T67821: Snap to Symmetry not updating

Added a missing depsgraph update.

Fix crash when opening files with missing armature libaries

In the case where the library is missing, the armature object is
replaced with an empty. This would crash the armature modifier.

Now we check if the armature object really is an armature or not.

Fix T67665 "Affect Alpha" in Texture Paint mode doesn't work as expected

The "alpha lock" check was missing for the smear and soften brush.
Added checks to make sure that the alpha values are kept the same.

Reviewed By: Brecht

Differential Revision: http://developer.blender.org/D5416

Fix T68073: Wacom Intuos 5S no pen pressure on Wayland

The issue is that wayland seems to impose a generic device naming scheme
when using Xwayland For example any table stylus will show up with the
following naming convention: xwayland-stylus:33

For this to work in blender, I had to modify how the identifier string
is extracted. I also renamed the two char pointers in the search
algorithm to be more logical.

Reviewed By: Brecht

Differential Revision: http://developer.blender.org/D5401

Fix T68122: Settings for Select root and Select tip in particle edit mode are missing

Do not hide the redo window for root/tip select anymore.

Revert "UI: remove checks for other popovers when switching menu"

This reverts commit 77616571292ab1eeb9805797bfdd4525cc306923.

This commit broke overlapping popovers. For example in the color
management tab it would be impossible to select and of the popover
alternatives as it would switch to the menu button under the cursor.

Fix T65837: "Zoom Axis" is not working on the node editor

We would not take into account the user "Zoom Axis" setting in certain
2D space viewports. In addition to this, the "Scale Zoom" didn't work
consistently in these spaces either.

Reviewed By: Brecht

Differential Revision: http://developer.blender.org/D5132