DITHER is on by default and dEQP precision tests turn this off, to avoid
dithering messing with the precision. The OpenGL backend didn't handle
glDisable(GL_DITHER) causing precision to be wrong on RGB565 on Linux
Intel.

BUG=angleproject:1492

Change-Id: I5e750fbe9df397e8b9a826979b14c84a2d72a00d
Reviewed-on: https://chromium-review.googlesource.com/382111Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>