In tiling GPUs, using two triangles means the tiles intersecting the
seam would be processed twice.

Bug: angleproject:4936
Change-Id: Ib10b77e6ab15bba932f0e4e970e10ed5a8399cdd
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2372623
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>