Added `abs` constructor to `Context`
Added `abs` constructor to `Context`

Closes https://github.com/PistonDevelopers/rust-graphics/issues/404

PistonDevelopers/graphics,mitchmindtree/graphics,stjahns/graphics,echochamber/graphics,aepsil0n/graphics,andrewcsmith/graphics,Vinatorul/graphics,bvssvni/graphics