Revert some previous commits about bad alpha

This reverts following commits: 4f842582217c2b2a5d6774e5e8b47ee34bb690e6
6757eb6bcba2243fde76cde45a6ec9a2dbf64905
c00b45c08eb72b872fe2d10e39a840d01db570bb
2f5d8f680c284a73499c1a444b24aa2d61c45b87
gamutcheck & overexposed: robust fix for alpha issues