GLES1: Fix Fog rendering The spec requires that: - The fog factor is clamped to [0, 1] - The fog only affects RGB, and not alpha Bug: angleproject:7598 Change-Id: Iaea2933b5ccfa79b7db0a3e74193b1c5c10f11e6 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3907881 Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Cody Northrop <cnorthrop@google.com>