Small UX fix. When hidding everything but the active face set with H,
the last stroke lotation center can be in a part of the model that is
hidden, so viewport navigation becomes confusing until you start a new
stroke on the visible face set. Now the viewport navigation rotation
center is updated to the active vertex when using a visibility operation
that uses it, so it always rotates using the visible face set as the
origin.
Reviewed By: jbakker
Differential Revision: https://developer.blender.org/D7137
7941ea02f8