Sculpt dyntopo: Removed triangle limit for PBVH_BMESH

* PBVH_BMESH now supports faces other then triangles;

* Dyntopo triangulates faces as it finds them.
- I looked into methods of preserving quads and failed to
find anything that worked well in practice; it actually
worked better to use topology rake to align triangles
into quads and then mark diagonal edges for later dissolving
then to try to preserve quads explicitly (I've not
implementated that here, that was research code).
- To avoid excessive cache-destroying loops over vertex-faces,
DynTopo flags which verts have non-triangle faces.

* PBVHTriBuf now builds edge buffers so we can avoid drawing
tesselation phantom edges.

* BMLog also now supports arbitrary faces. It still does not
support edges though.

TODO:

* Fix vcol cell shading mode
* Make sure indexed drawing works

Sculpt dyntopo: Another fix for faceset boundary handling

* Turned out pbvh->cd_faceset_offset wasn't being updated
correctly.

Sculpt dyntopo: Another small fix

A few small fixes for last commit

Sculpt dyntopo: Face set boundaries are now presered with dyntopo

* Face set boundaries are now preserved on dyntopo remeshing.
* MDynTopoVert->flag now has a DYNVERT_FSET_BOUNDARY flag
in addition to DYNVERT_BOUNDARY.
* Instrumented uiBut with ASAN poison regions to hopefully
find the super evil memory corruption bug that's been driving
me insane. It's frustratingly intermittent. There are five
poison regions.

Sculpt dyntopo: Dyntopo now handles mesh boundaries in a more
intelligent way.

Add back hackish fix for clang bug. I should probably
file a bug report with the clang people.

Sculpt dyntopo: Split off dyntopo stuff from pbvh_bmesh.c into a new file, dyntopo.c.

Sculpt dyntopo:

* Prototyped a threaded bmesh->Mesh converter function. It's about
20-30% faster. Currently disabled.
* Tried to fix more of the bugs when stepping between sculpt and
other undo step types in the global undo stack.

Merge branch 'master' into temp_bmesh_multires