Implement textureProjGrad and textureProjGradOffset. BUG=angle:564 Change-Id: Ice6a3715b0fda13324b3162940de875c932627c9 Reviewed-on: https://chromium-review.googlesource.com/186992 Tested-by: Nicolas Capens <nicolascapens@chromium.org> Reviewed-by: Shannon Woods <shannonwoods@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>