Simplify grvx::approx_angle_between_vectors signature

Rather than taking the x and y values separately (ax, ay, bx, by),
simply take two vec<N*2>'s (a, b), where the x's are in a.lo and the
y's in a.hi.

Bug: skia:10419
Change-Id: I21d659c79247ccb625351c20b93c550d0afffe79
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/339458
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
2 files changed