Blender Git Loki

Fix Boundary Brush symmetry with no constant falloff

When using constant falloff symmetry was working fine because the same
deformation is applied twice on the same vertices. When using no
constant falloffs, the deformation is different between symmetry passes,
so vertices need to be separated by symmetry areas to get the right
deformation from its symmetry pass without being overwriten by the next
one.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D8541