Make Frame::get_mask easier to understand
Make Frame::get_mask easier to understand

tanderegg/gfx,tempbottle/gfx,Bastacyclop/gfx,aepsil0n/gfx-rs,bvssvni/gfx-rs,red75prime/gfx,mitchmindtree/gfx-rs,JohnColanduoni/gfx,alexcrichton/gfx,ColinKinloch/gfx,tanderegg/gfx,gfx-rs/gfx-rs,sectopod/gfx-rs,davll/gfx,JohnColanduoni/gfx,ColinKinloch/gfx