Fix sysgpu_backend build option

[Tatskaari]

The sysgpu backend build option was treating the build option as the wrong enum type. This type is defined in build.zig and is exposed through the generated build_options struct. This is really nasty, but I think this is the correct way to do this.

The generated code for @import("build_options") for reference:

pub const want_mach: bool = true;
pub const want_core: bool = true;
pub const want_sysaudio: bool = true;
pub const want_sysgpu: bool = true;
pub const @"build.SysgpuBackend" = enum (u3) {
    default = 0,
    webgpu = 1,
    d3d12 = 2,
    metal = 3,
    vulkan = 4,
    opengl = 5,
};
pub const sysgpu_backend: @"build.SysgpuBackend" = .opengl;
pub const @"build.Platform" = enum (u3) {
    wasm = 0,
    linux = 1,
    windows = 2,
    darwin = 3,
    null = 4,
};
pub const core_platform: @"build.Platform" = .linux;

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[slimsag]

This being named build_options.@"build.SysgpuBackend" looks weird and wrong to me.

We have other options in build_options that don't have to be written that way, for example here you can see we have build_options.want_sysaudio as a boolean: https://github.com/hexops/mach/blob/main/src/main.zig#L18

I think we do something similar for core.PlatformType: https://github.com/hexops/mach/blob/main/build.zig#L48

Can you look into making it work similar to that?

[slimsag]

I think the solution might be writing something like:

const backend_type: sysgpu.BackendType = switch(build_options.sysgpu_backend) {
    // ... all the cases ...
};

And then don't have a backendFromBuildOption function at all.

[Tatskaari]

Ah yeah that would be nicer. I've updated the PR. I've just learnt the .value syntax so it didn't occur to me that you can avoid specifying the type entirely via inference here.

[slimsag]

Looks great, thank you for fixing this!