Seperate enabling PBVH_BMESH from enabling DynTopo:
* Created a new option to globally disabled
DynTopo.
* The DynTopo panel header now reads "Dynamic Mode",
to hopefully signal that turning on PBVH_BMESH is
a seperate step from enabling or disabling DynTopo
itself.
* The first checkbox in the panel is "DynTopo" so it
should be clear enough (it's on by default, with multiple
layers of file versioning checks).
PBVH_BMesh's undo system:
* CD_MESH_ID layers are now permanently saved once
they are created (by default they are not). This
fixed a *lot* of bugs:
Before this the undo system had to save maps between
mesh indices and mesh IDs on transitioning
between sculpt and global undo steps. This was
extremely error prone, and it simply wasn't possible
to cover all of the corner cases
* Note that there is still an odd bug where the first
global undo push after a sculpt step gets ignored,
I dunno what's up with this.
* Dyntopo undo should be nearly (hopefully completely)
bug-free after this commit.
C++20
* Made a few small changes to get blender to compile
with c++20. std::result_of was removed, had to
replace a couple of usages of it with std::invoke_result.
* I'm planning to do some design studies on rewriting
sculpt into C++.
* I strongly suspect we are going to need C++20'a new
concepts feature if we move sculpt into C++.
I'm planning to do some design studies on how
that might work.
