Implementations of sdl2::gfx::primitives::ToColor has undefined behavior

[belak]

https://github.com/Rust-SDL2/rust-sdl2/blob/7fd7ee4/src/sdl2/gfx/primitives.rs#L21

Hello! I was trying to get some of the basic drawing functions working for an SDL 1.2 library and I was looking here for sample code. However, I was having issues with some colors not working. After investigating I found that ToColor uses UB because the layout of tuples in memory isn't guaranteed (so mem::transmute won't always work).

I think most uses of transmute can be replaced with either u32::to_be_bytes or u32::from_be_bytes to fix the issue. Ideally, there could be a single color type for the whole crate, but that might be too complex.

Here's a sample test which fails for me:

#[test]
fn test_color() {
    let r: u8 = 255;
    let g: u8 = 0;
    let b: u8 = 255;
    let a: u8 = 255;

    assert_eq!((r, g, b, a), 0xFF00FFFFu32.as_rgba());
}

It's not a great test, but it shows the issue I'm running into.