Add xxxx32 pixel formats

[Rua]

Since SDL 2.0.5, there are four new pixel formats for "array" formats: RGBA32, ARGB32, BGRA32 and ABGR32. These map to the existing 8888 "packed" formats, but in different ways per platform so that the in-memory order of the colours is always the same regardless of platform.

The Rust version of SDL is missing these. SDL_pixels.h defines these using conditional compilation, and I'm not sure how that should be done in Rust, or I would make a PR.

[Cobrand]

The Rust version of SDL is missing these. SDL_pixels.h defines these using conditional compilation, and I'm not sure how that should be done in Rust, or I would make a PR.

But that's only used internally by SDL, right? Wedon't have to make conditionals in the Rust code.

For the PR, I think youyr best bet would be to look here: https://github.com/Rust-SDL2/rust-sdl2/blob/master/src/sdl2/pixels.rs#L202 , add those values and then see where PixelFormatEnum is used in the code exactly. I don't think you'll have to change much, but I might forget something here, so let's be extra careful and see where all those variants are used in the code.

[Rua]

They're available to users of SDL and are documented: https://wiki.libsdl.org/SDL_PixelFormatEnum. We must assume that users of Rust SDL will want to use them, and I'm one of those users.

What is different is that they don't have unique enum values, but are simply aliases of existing formats and have the same enum value as the other format. Which formats they are aliases of differs depending on the endianness. RGBA32 is the same as RGBA8888 on a big-endian platform, but the same as ABGR8888 on a little-endian platform.

This is a bit of a problem for Rust, because enum values must be unique. Adding these new enum variants, and assigning them a number that's the same as another variant, will result in an error. I did a quick search and found that associated constants may work. Something like this:

#[cfg(target_endian = "big")]
impl PixelFormatEnum {
    pub const RGBA32: PixelFormatEnum = PixelFormatEnum::RGBA8888;
}

I can't say if this works exactly as an equivalent to a true enum variant, but it's probably better than nothing.

[Cobrand]

I understand now, the true problem comes from the fact that 2 enum variants can't have the same value. Makes sense. I'm fine with your proposal of the associated const, just make sure that nightly is not required to use associated consts (I'm not sure if it has been stabilized, but it was in nightly at some point for sure). In any case, if that doesn't work, the alternative would be to have a static method in the impl rgba32() -> i32 { ... } or something.

[Rua]

Associated constants are stable since Rust 1.20: https://rust-lang-nursery.github.io/edition-guide/rust-2018/trait-system/associated-constants.html.

[mnmaita]

@Cobrand Looks like #827 solved the issue.