Removed mutability requirement from subsystem.clone and from subsystem.sdl
Removed mutability requirement from subsystem.clone and from subsystem.sdl

vadixidav/rust-sdl2,Rust-SDL2/rust-sdl2,Rust-SDL2/rust-sdl2,AngryLawyer/rust-sdl2,oldmanmike/rust-sdl2,Rust-SDL2/rust-sdl2,Cobrand/rust-sdl2,Cobrand/rust-sdl2,Cobrand/rust-sdl2