Thank you Nicolas,
I verified the fix, it works.
I took nicolas' change and build a windows pharo vm. changing the bit depth
to 8bit littleendian (-8) works now.
But there are two more issues (not related to this fix).
1. Changing Display depth to 4/8/16 shows horrible rendering artifacts
(those do not occur on squeak)
2. Since refactoring some parts out of DisplayScreen into MorphicUIManager you
can not change the depth from -XX to XX
Display newDepth:8
has no effect if current display depth is -8.
(This happens on squeak too, verified in Squeak 4.4/Squeak 4.5).