video/out/gpu/context: convert --gpu-api to object settings list

[guidocella]

This follows up 96e1f1dfa5 which converted --gpu-context, and has the same advantages as listed there.

Unlike with --gpu-context auto can be used anywhere in the list, e.g. --gpu-api=d3d11,auto works.

I wanted to use the list of GPU contexts as the description in get_type_desc(), but there is no talloc context to allocate it to, so I set a print_help_list to print them. The printed lines don't being with spaces because otherwise they are added to the completions by etc/_mpv.zsh.

[github-actions[bot]]

Download the artifacts for this pull request:

Windows

* [mpv-i686-w64-mingw32](https://nightly.link/mpv-player/mpv/actions/artifacts/1668988979.zip) * [mpv-x86_64-w64-mingw32](https://nightly.link/mpv-player/mpv/actions/artifacts/1668991400.zip) * [mpv-x86_64-windows-msvc](https://nightly.link/mpv-player/mpv/actions/artifacts/1669042708.zip)

macOS

* [mpv-macos-12-intel](https://nightly.link/mpv-player/mpv/actions/artifacts/1668984504.zip) * [mpv-macos-13-intel](https://nightly.link/mpv-player/mpv/actions/artifacts/1668983705.zip) * [mpv-macos-14-arm](https://nightly.link/mpv-player/mpv/actions/artifacts/1668981478.zip)

[na-na-hi]

The printed lines don't being with spaces because otherwise they are added to the completions by etc/_mpv.zsh.

This can be fixed by printing the GPU API names first instead of context names, right? This is the current behavior of --gpu-api=help. I think this is a bad reason to remove spaces just because some outside scripts can't handle it.

[guidocella]

With the APIs first they get completed twice and the last contexts are interpreted as descriptions:

mpv --gpu-api=<Tab>
auto    -- auto
opengl  -- drm
vulkan  -- displayvk
auto    opengl  vulkan

Actually completing them works though so it's acceptable.

[Dudemanguy]

It's a bit less nice than --gpu-context since using completing --gpu-api here automatically adds a space after the completion. But this is better than the current status quo of "doesn't complete at all" so I'm fine with it if you don't want to investigate further.

[guidocella]

A space is added after completions with only 1 result, --gpu-api doesn't do anything different.

[Dudemanguy]

Ah my bad. I was typing wayland and foolishly thought that was a full completion.