Early fragment test optimization has to be disabled if alpha to coverage
is enabled. This test that the intersection between alpha to coverage
test and early fragment test optimization.

Bug: angleproject:5082
Change-Id: I628efc2fae7c998467e1daff4167f16fe0c28142
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2422899
Commit-Queue: Charlie Lao <cclao@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>