Tracking: Forward compatibility code for distortion models

Allows to open newer files in older Blender after new distortion model
has been added.

It will behave as if this is a polynomial model with all 0 coefficients
which are then being refined and assigned explicitly after solving the
motion.

Tracking: Fix missing distortion update on focal length change

Fix crash using object.to_mesh() when in edit mode

The root of the issue was caused by mesh which was a result of to_mesh()
had the same edit_mesh pointer as the input object, causing double-free
error.

This fix makes it so result mesh does not have edit mesh pointer.
Motivation part behind this is to make the result of to_mesh() to be
somewhat independent from the input.

Differential Revision: https://developer.blender.org/D7361

Tracking: Fix (un)distortion happen in single thread

Need to communicate available number of threads to the camera
intrinsics implementation, otherwise default value of 1 is used.

Must have been single-threaded for a very long time.

Libmv: Use static scheduler for threading

For a real-world distortion the payload is quite uniformly
distributed across scanlines. Surely, in the corners more
iterations of minimizer is needed, but that happens in threads
without scheduling overhead.

Tracking: Fix slow undistored display

TH distortion model was not cached properly, making it so frame is
undistorted on every redraw.

Fix T75297: Apply base inflates meshes with Simple subdivision

Sculpt: Store explicit value for multires sculpt level

Allows to know what level sculpting has been done after the value has
been changed in the MultiresModifierData.

No functional changes, just preparing code to have everything needed
for propagation undo.

Differential Revision: https://developer.blender.org/D7307

Sculpt Undo: Allow Geometry undo step to be non-exclusive

Before this change it was not possible to have base geometry
and grid coordinates to be stored in the same undo step.

Differential Revision: https://developer.blender.org/D7298

Sculpt: Give a brief explanation of undo stack

Should make it a bit more clear overview of what is going on in this
module. While some of the details might still be missing, having some
sort of top-level overview is better than nothing.

Differential Revision: https://developer.blender.org/D7300