This implements instanced path rendering.

BUG=angleproject:1382

Change-Id: I4654251882975d707b22c7bb522af14ae34eb1a1
Reviewed-on: https://chromium-review.googlesource.com/355401Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Sami Väisänen <svaisanen@nvidia.com>