Sculpt: Line gestures and Mask Line tool

This adds support for line gesture to SculptGestureContext and
implements a Mask Line tool, which affects everything to the right of a plane
defined by the straightline gesture.

For this to work, a new WM_gesture_straightline_oneshot_modal is needed
which only runs exec when the gesture is over.

Added as experimental as it does not have icon.

Reviewed By: Severin

Differential Revision: https://developer.blender.org/D8722

Fix Face Set overlay rendering wrong color with modifiers

face_set_color_default was not copied when copying meshes, so when
extracting the colors in the draw cache it was using a wrong default
color ID.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D8957

Cleanup: CMake config: set options in alphabetical older

No functional change.

Revert "Generate Xcode scheme files during configuration."

This reverts commit 5eb5978043c8e7036e15572e.

The change was problematic:
*Advanced users may prefer managing schemes manually and adding only
a few that they need, instead of one for every library/executable
Blender builds.

*If CMake creates schema files, it overwrites the changes a developer
made every time CMake is run. If Xcode creates scheme files, it keeps
settings/ environment variables intact.

If someone runs CMake very frequently, pass
`-DCMAKE_XCODE_GENERATE_SCHEME=ON` to `cmake` while configuring.

CMake: Remove duplicate WITH_GMP options.

I didn't realize there were more duplicates while committing
{rBf1fee433be92}.

Original was added in {rB83f8223543f58c3b0881a03b6e9ddffff91}.
Duplicate was added in the merge {rB9e09b5c418c0a436e3c84ccf}.

Ref D8822

macOS: Remove ASan debug flags from release config.

Setting `PLATFORM_LINKLIBS` is not required for clang, compiler and
linker flags are enough. Note that the change made in
{rBa4c5811e2127}) to `platform_apple.cmake` (appending to
`PLATFORM_CFLAGS`) has not been reverted. platform file shouldn't be
overwriting the flags.

`PLATFORM_LINKFLAGS` is overwritten by `platform_apple.cmake`, so no
point in setting it. Fixing that like `PLATFORM_CFLAGS` is out of the
scope of this change.

`PLATFORM_LINKFLAGS_DEBUG` has been replaced with generator expression
to include RelWithDebInfo and MinSizeRel build types also.

Revert "remove alembic code"

This reverts commit cae7e63fca81f9fd1bd9110a879b1e1309ae2d4c.

further cleanup, remove unnecessary friends from Mesh

Fix T81205: Grease pencil material slots don't auto-erase void slots above the first material

Fix T81168: 2.8x Crash when loading 2.79 file.

Object in the 2.7x file is on no layer at all, while this is not really
expected nor considered valid, better to cope with the case gracefully
than crash.

To be backported to 2.90 and 2.83.