Patch for #4620 - ensure posDetectSurfaces is invalidated on slice change

[lordofhyphens]

Invalidate posDetectSurfaces during make_perimeters() when posSlice would also get invalidated.
Can't use invalidate_step() as that invalidates the perimeters as well.

Fixes #4620

[AppVeyorBot]

✅ Build Slic3r 1.3.0-master-2222 completed (commit 6576c81a74 by @lordofhyphens)

[lordofhyphens]

Also have a regression test added to the CATCH test suite.

[AppVeyorBot]

✅ Build Slic3r 1.3.0-master-2224 completed (commit 86adaa2c9d by @lordofhyphens)

[VanessaE]

I can't comment on the code itself, but the behavior is spot-on. lgtm.

(no idea why CI barfed on the OSX build, but no problems here on Debian testing/Buster)

[alranel]

I'd turn this->state.invalidate(posSlice); into this->invalidate_step(posSlice); like we were doing before the bisected commit. In order to do this safely, I'd move set_started(posPerimeters) after the if (this->typed_slices) { block.
I think that state.invalidate() should be never called directly, except from invalidate_step() which ensures dependencies are invalidated too

[lordofhyphens]

Done.

[AppVeyorBot]

✅ Build Slic3r 1.3.0-master-2234 completed (commit 4ca602b32b by @lordofhyphens)