Retain FileContext to lookup $vp{rtd} variables after compilation (fixes #949).
Rewrote tessellation used for NefPolyhedron to PolySet conversion. Should fix #1033
Tagged stlpngtest_bad-stl-wing as a bug
Merge pull request #1037 from openscad/issue949

Use FileContext when evaluating $vp{rtd} variables (fixes #949).