[gfx] Adding additional vec2 utilities

These utilities simplify the implementation of gesture interpretation. No
effort is made to provide a complete family of operations; only operations
used in https://fuchsia-review.googlesource.com/278623 are added.

Test: fx run-test math_unittests
Change-Id: I3323dbd9e515edda81f0903ef24e035395fe9e02