Fix stack overflows in face traversal and STEP reference cycles, set the version to 1.7.0 (#486, #487)

[dsn27]

Two crash fixes for 1.7.0 and the version bump.

#486: collect connected faces without recursion

Make3D.extractConnectedFaces, BRepOperation.extractConnectedFaces and BRepSelfIntersection.collectFaces walked the faces of a shell with a recursive depth-first search that nested once per connected face. A DXF mesh of about 7000 triangles (sewing) or a boolean operation on a shell with some ten thousand faces overflowed the stack, and a StackOverflowException terminates the process.

The three traversals now use an explicit stack. They visit the faces in the same order as the recursion did, so the shells come out unchanged.

#487: cut reference cycles in the STEP import

ImportStep.CreateEntity creates referenced entities recursively. It only knew that an entity was created once creation had finished. A reference cycle therefore re-entered CreateEntity for the same entity until the stack overflowed (the monitor is re-entrant). An example of such a cycle is a MAPPED_ITEM whose REPRESENTATION_MAP maps the representation that contains the mapped item.

• Same thread: each item records the thread that is creating it. When that thread reaches the item again, the reference closes a cycle. That reference becomes null, and it is reported as "cyclic reference" in the import problems. The rest of the file is imported.
• Parallel import: with StepImport.Parallel (the default), two threads can enter a cycle from different ends and then wait for each other's lock forever. This is the limitation noted in the issue. A thread that has to wait for an item now follows the chain of threads that wait for each other. When the chain leads back to an item the thread is creating itself, it cuts the cycle the same way instead of waiting.
• DEBUG builds: the early return for an entity that another thread created in the meantime no longer leaves the DEBUG-only definition stack unbalanced.

Version

<Version> is set to 1.7.0. A minor bump fits because the release adds public API since 1.6.0:

• IJsonProgress, DXF.IDxfProgress and the progress overloads (Show a progress bar during file open #367)
• Text.ClearFontCache (Public way to release the glyph display lists of FontCache (renderer switch, graphics device loss) #494)
• ProjectedModel.InvalidateDisplayLists (Lightweight way to make a ProjectedModel rebuild its display lists #496)
• the PaintTo3DOutOfMemory constructors (PaintTo3DOutOfMemory cannot be thrown by IPaintTo3D implementations outside CADability #495)

This PR adds no public API.

Tests

• DeepFaceTraversalTests runs two operations on a thread with a 256 KB stack:

  • it sews a grid of 3200 triangles;
  • it subtracts a box from a prism with 3000 sides.

  Without the fix, both crash the test host with a stack overflow.

• StepCyclicReferenceTest imports issue101.stp in two variants, sequentially and in parallel:

  • with a representation that maps itself: without the fix, the test host crashes with a stack overflow;
  • with two representations that map each other: with only the same-thread check, the parallel import does not finish.

• Local results on Linux:

  • CADability.Tests: 636 passed, 19 skipped, 0 failed.
  • CADability.ImportTests: 85 passed, 5 skipped, 0 failed.

Closes #486, closes #487.

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