Gfx: specifically use DeviceGray instead of DefaultGray for softmasks
This is another spin-off from !643 (merged) .
The reason it was so far part of !643 (merged) is that it is seldom triggered without explicitly setting DefaultGray externally, and thus only showed up there. However, this might be a fix in its own right.
@aacid Could you run your test suite on this one?
Thx for reviewing