Move out generic math parts out of the native barrier implementation

In order to reuse some vector math for pointer confinement, move out
those parts to its own file, introducing the types old types
"MetaVector2" and "MetaLine2" outside of meta-barrier-native.c, as well
as introducing MetaBorder which is a line, with a blocking direction.

https://bugzilla.gnome.org/show_bug.cgi?id=744104