This adds a property that checks the normals of each vertex against the
view direction to decide if they should be maskes (similar to the
"Front Faces Only" option works for brushes.
It is only implemente for Lasso Mask as Box Mask uses the
View3D_OT_select_box operator. It should probably be refactored into its
own operator int a later patch as it is not really a selection, so it
both can include more sculpt mode related funcitonality.