Added `EllipseBorderContext` and `EllipseBorderColorContext`

Closes https://github.com/PistonDevelopers/rust-graphics/issues/467
Closes https://github.com/PistonDevelopers/rust-graphics/issues/470
Merge pull request #484 from bvssvni/master

Added `EllipseBorderContext` and `EllipseBorderColorContext`