The simple subdivision as a type only causes issues like no-continuous
normals across edges, inability to reliably switch the type and things
like this.
The new subdivision operators supports wider variety of how to add
details to the model, which are more powerful than a single one-time
decision on the subdivision type.
The versioning code is adjusting topology converter to specify all
edges as infinitely sharp. The reason for this (instead of using
settings.is_simple) is because in a longer term the simple subdivision
will be removed from Subsurf modifier as well, and will be replaced
with more efficient bmesh-based modifier.
This is finished up version of D8436.
Differential Revision: https://developer.blender.org/D9350
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.
The initial code from earlier from today didn't really work reliable
since it is not possible to apply virtual modifiers but not the real
multires one (in a situation like mesh with shapekeys and multires).
New code uses less memory and has better performance for the case
when there are actual modifiers leading the multires. The case when
there is only multires will not be as performant as possible at this
moment.
This change fixes artifacts produced by these operations.
On a technical aspect this is done by porting all of the operations
to the new subdivision surface implementation which ensures that
tangent space used to evaluate modifier and those operations is
exactly the same (before modifier will use new code and the operations
will still use an old one).
The next step is to get sculpting on a non-top level to work, and
that actually requires fixes in the undo system.
