Added ZR for zero rect

changed wording to match zero-vector returns in other functions.

Fixed doc refrince to R(0,0,0,0) in Rect.Intersect() to be a ZR and for it to return a ZR.

Also ran gofmt...

Added the ZR for rectangle for both utility and consistensy with pixel.ZV

Fixed link to examples repo in CONTRIBUTING.md

Added TriangleData benchmarks and improvements

Fix vertex race

Setting position in correct order

Avoiding floating point rounding errors

why 2 loops and assign twice? one is enough

Add line geometry