workflows/lint: add clang-format on changed files

[kasper93]

The entire codebase is not ready to be clang-formatted and probably never will be, but we can at least check if the changes in new pull requests follow our coding style.

[github-actions]

Download the artifacts for this pull request:

Windows

• mpv-i686-w64-mingw32
• mpv-x86_64-w64-mingw32
• mpv-x86_64-windows-msvc

macOS

• mpv-macos-13-intel
• mpv-macos-14-arm
• mpv-macos-15-arm

[sfan5]

Has anyone tried to verify how well this matches our code style by running it on existing files?
If not I will.

[sfan5]

Here's it applied to a few big files: sfan5@bf400f2

As I feared, either the config is not fitting or clang-format is just too opinionated to be applied to all new or changed code.
In my experience it's the latter and you either up with a sea of // clang-format on|off or you submit yourself to the linter compromising on readability.

just look at this

    .defaults =
        &(const struct demux_opts){
                                  .enable_cache       = -1,  // auto
            .max_bytes          = 150 * 1024 * 1024,
                                  .max_bytes_bw       = 50 * 1024 * 1024,
                                  .donate_fw          = true,
                                  .min_secs           = 1.0,
                                  .min_secs_cache     = 1000.0 * 60 * 60,
                                  .seekable_cache     = -1,
                                  .index_mode         = 1,
                                  .mf_fps             = 1.0,
                                  .access_references  = true,
                                  .video_back_preroll = -1,
                                  .audio_back_preroll = -1,
                                  .back_seek_size     = 60,
                                  .back_batch =
                {
                    [STREAM_VIDEO] = 1,
                    [STREAM_AUDIO] = 10,
                }, .meta_cp = "auto",
                                  },

[kasper93]

@sfan5: Fixed. Any other suggestions?