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

Sculpt Undo: Fix multires undo for interleaved nodes

Made it so grids array is properly allocated when first node in the
undo list does not contain grid data.

Differential Revision: https://developer.blender.org/D7299

Sculpt Undo: Refactor Geometry undo nodes

Made it so there is a single UNDO node in the list which has
both original and modified mesh state.

Makes it easier to achieve "interleaved" undo nodes stored in
the undo step (as opposite of either storing geometry or other
data).

Should be no functional changes, just preparing for an upcoming
work to support undo of operation like Apply Base.

Differential Revision: https://developer.blender.org/D7290

Subdiv: Split evaluation begin+refine into separate steps

Actually, begin will do the entire initialization.

Refine will only refine if there is a topology refiner associated
with the Subdiv descriptor.

Allows to refine Subdiv to new coarse positions without touching
displacement evaluation. Will be needed to update SubdivCCG during
sculpt undo.

Multires: Fix unwanted assignment of sculpt session pointers

Might have happened when Apply Base is used in sculpt mode.

In practice this probably was fine, since the operator tags object
for update, so the pointers will be restored back to what they should
be.

CMake: Fix compilation with Xcode generation on Xcode 11.4

Need to give correct SDKROOT.

CMake: Remove support of Xcode prior to 8.2

The specific goal of this change is to get rid of separate code paths
for older and newer Xcode versions.

The version 8.2 is picked since it's the latest version which runs on
macOS 10.11 (which is our current deployment target). If that turns
out too new for some reason the alternative would be to require Xcode
version 5.

CMake: Fix macOS SDK detection with latest Xcode and macOS

Happens on macOS 10.15.4 and Xcode 11.4.

The reason of failure is caused by following factors:

- xcodebuild reports full semantic macOS SDK version 10.15.4
- The actual SDK file path will only include major and minor part
of the version (10.15, MacOSX10.15.sdk)
- Previous CMake code of ours expected direct match between SDK
version and file path.

The solution is to make our detection code a bit more flexible and
additionally check for major.minor macOS SDK version in the path.