Fix issue 13154 alphaSourceBlendBits8

This fix issue https://pharo.fogbugz.com/f/cases/13154/BitBlt-bug-in-alphaSourceBlendBits8
The issue was reported by Nicolai Hess on mantis
It was fixed and integrated in VMMaker (Squeak) and VMMaker.oscog branches by Tim Rowledge, and is in use for more than 6 months now.

I don't see how this could cause unstability of any kind, so I strongly suggest integration before 3.0 release.