Blender Git Loki

Fix T71759: Sculpt/Vertex/Weight Paint Brush Size Gets Undone After Undoing a Stroke.

Add code preserving scene's toolsettings accross undo.

IDPointers are dealt with special care, we try to keep existing ones for
some (like brushes) when possible.

Note that this covers ToolSettings, Brushes and Palettes currently.

I'm not especially happy about how this new code mixes with existing
'foreach_id' one, in particular in scene. But cannot think of a better,
more generic way to do it currently.

Maniphest Tasks: T71759

Differential Revision: https://developer.blender.org/D9311