Render pixels concurrently

Iterate Rects to produce the image

Add Rect
Add RectIter
Rect.iter() -> RectIter
Implement next for RectIter, iterates all points in the rect

Remove blank lines
Merge branch 'master' of https://github.com/porglezomp/rust-raytracer

Broke commit history
Conflicts:
	src/main.rs