A few things were failing when trying to use
GL_SAMPLE_ALPHA_TO_COVERAGE:
- The alpha test should not be skipped when
  GL_SAMPLE_ALPHA_TO_COVERAGE is set, otherwise
  no computation is performed.
- The alpha reference value was wrong
  (the alpha-to-coverage ramp is centered around it)

Change-Id: Ib9849868d267ca49fb3f0112192af8f207d34307
Reviewed-on: https://swiftshader-review.googlesource.com/8308Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>