Continuing the refactors described in T93602, this commit moves
the face dot tag set by the subdivision surface modifier out of
`MVert` to `MeshRuntime`. This clarifies its status as runtime data
and allows further refactoring of mesh positions in the future.
Before, `BKE_modifiers_uses_subsurf_facedots` was used to check
whether subsurf face dots should be drawn, but now we can just check
if the tags exist on the mesh. Modifiers that create new new geometry
or modify topology will already remove the array by clearing mesh
runtime data.
Differential Revision: https://developer.blender.org/D14680
