Fix issue 12539 fast-up BitBlt alpha blending rules

This is not a bugfix but an enhancement, so it can be postponed to Pharo4.0 if you wish.
Though, the changes are well localized (handle channels 2 by 2 rather than 1 by 1 in 3 methods).
See https://pharo.fogbugz.com/f/cases/12539/Faster-bitblt-alphabending-rules
or http://bugs.squeak.org/view.php?id=7803