Fix variable type issue with Rust 1.6
Fix variable type issue with Rust 1.6

vadixidav/rust-sdl2,Rust-SDL2/rust-sdl2,AngryLawyer/rust-sdl2,Cobrand/rust-sdl2,Cobrand/rust-sdl2,notkild/rust-sdl2_image,Machtan/rust-sdl2_image,Rust-SDL2/rust-sdl2,Cobrand/rust-sdl2,xsleonard/rust-sdl2_image,crumblingstatue/rust-sdl2_image,Rust-SDL2/rust-sdl2