Merge pull request #521 from bvssvni/master

Made rust-image compile under gl flag
Merge pull request #523 from bvssvni/master

Added `viewport`
Merge pull request #525 from bvssvni/master

Made code nicer
Merge pull request #527 from bvssvni/master

Moved `gl::BindBuffer` outside unsafe block
Fix issue #304
Merge pull request #529 from Potpourri/master

Fix issue #304 in piston
Replaced `Stroke`, `Fill`, `Clear` with `Draw`

Closes https://github.com/PistonDevelopers/rust-graphics/issues/532