Blender Git Loki

Add ability to constrain knife cut angle to 0, 45, or 90 degrees
Notes:
- uses 'c' as toggle key in and out of angle snapping
(aka constrain) mode
- when in angle snapping mode, after first click, will
constrain line to nears 0, 45, or 90 degree line
- constraint is in screen space, not model space,
so will usually be used in axis aligned view