Internal pre-bridge supports

[boromyr]

There is a fundamental lack in PrusaSlicer, namely the ability to support internal bridges and "internal top layers". It's all much more evident from the comparison screenshots with SuperSlicer, which has this function, while in Prusa these layers run without having any underlying support. I used a 1% fill to make the problem more obvious.

[rtyr]

Duplicate of #569. Closing.