Implemented `AddBorder` for `BevelRectangleContext`
Implemented `AddBorder` for `BevelRectangleContext`

Closes https://github.com/PistonDevelopers/rust-graphics/issues/498

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