This will be useful for line segment rasterization.

Bug: angleproject:2598
Change-Id: I84912c976665ecb32903181fa820b8db88786a28
Reviewed-on: https://chromium-review.googlesource.com/1127299
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>