Update drawing logic (fixes #1035).

- Draw negative part of the axis with stippled lines
- Calculate contrast color for axis based on background color
- Disable lighting for axis / crosshair
Fix crosshair at the center of the display (fixes #1025).
Render crosshair a bit less intrusive.
Merge pull request #1038 from openscad/issue1024+issue1035

Update drawing logic
Retain FileContext to lookup $vp{rtd} variables after compilation (fixes #949).