RendererGL: only support ES3 when GLSL packing builtins are available
RendererGL: only support ES3 when GLSL packing builtins are available

BUG=angleproject:1188

Change-Id: I07638c52ef09ffc96d91eec25c32dd905207c49f
Reviewed-on: https://chromium-review.googlesource.com/309650
Tryjob-Request: Corentin Wallez <470c533e86f4f848c98c7e11fffc2d043494a401@chromium.org>
Reviewed-by: Geoff Lang <b6fc25fe0362055230985c05cbfa8adb741ccc0f@chromium.org>
Reviewed-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Tested-by: Corentin Wallez <470c533e86f4f848c98c7e11fffc2d043494a401@chromium.org>

mlfarrell/angle,MSOpenTech/angle,ppy/angle,ppy/angle,ppy/angle,ecoal95/angle,MSOpenTech/angle,mikolalysenko/angle,ecoal95/angle,mikolalysenko/angle,mlfarrell/angle,mlfarrell/angle,domokit/waterfall,domokit/waterfall,ecoal95/angle,ecoal95/angle,mlfarrell/angle,ppy/angle,MSOpenTech/angle,ecoal95/angle,mikolalysenko/angle,MSOpenTech/angle,mikolalysenko/angle