Removed `Triangle`
Revert "Removed `Triangle`"

This reverts commit e109e250253fa10f04e62b5761c2b061d03edb33.
Merge branch 'master' of https://github.com/bvssvni/rust-graphics
Merge branch 'master' of https://github.com/bvssvni/rust-graphics
Merge branch 'master' of https://github.com/bvssvni/rust-graphics
Merge branch 'master' of https://github.com/bvssvni/rust-graphics
Merge branch 'master' of https://github.com/PistonDevelopers/rust-graphics
Renamed type alias `PixelRectangle` to `SourceRectangle`
Merge pull request #415 from bvssvni/master

Added `RelativeSourceRectangle` trait
Merge pull request #414 from Coeuvre/master

Renamed type alias `PixelRectangle` to `SourceRectangle`