The mirror modifiers merge option caused unnecessary re-ordering
to the vertex array with original vertices merging into their copies.
While this wasn't an error, it meant creating a 1:1 mapping from input
vertices to their final output wasn't reliable (when looping over
vertices first to last) as is done in
BKE_editmesh_vert_coords_when_deformed.
As merging in either direction is supported, keep the source meshes
vertices in-order since it allows the vertex coordinates to be extracted.
NOTE: Since this change introduce issues for some cases (e.g. bound
modifiers like SurfaceDeform), this change is only applied to newly
created modifiers, existing ones will still use the old incorrect merge
behavior.
Reviewed By: @brecht
Maniphest Tasks: T93321, T91444
Differential Revision: https://developer.blender.org/D13355
Because mesh vertex and face normals are just derived data, they can
be calculated lazily instead of eagerly. Often normal calculation is
a relatively expensive task, and the calculation is often redundant
if the mesh is deformed afterwards anyway.
Instead, normals should be calculated only when they are needed. This
commit moves in that direction by adding a new function to tag a mesh's
normals dirty and replacing normal calculation with it in some places.
Differential Revision: https://developer.blender.org/D12107
These functions do not change their source or input mesh, so it can
be passed with const, which means in one case that a function doesn't
have to be responsible for freeing its argument mesh, which is a clearly
better separation of concerns.
