Update Cargo.toml to allow specifying using "metal" gfx backend

[tigleym]

Building the project on MacOS requires the user to change the gfx backend to use "metal" by changing the following line in Cargo.toml:

features = ["vulkan"] => features = ["metal"]

Without this, the user won't be able to compile the game. This issue should investigate adding an option to allow compiling the game by specifying which backend should be used. Perhaps something like:

cargo build --features metal    // defaults to "vulkan"