vo_libmpv: introduce 'gpu-next' render backend

[sparky3387]

This commit adds a new rendering backend for the libmpv render
API, named 'gpu-next'. It is designed as a successor to the legacy
'gpu' backend for API users.

The new backend is built entirely on top of libplacebo, offloading all
complex rendering tasks like scaling, colorspace conversion, OSD, and
tone-mapping to it. This aligns the render API with the core mpv
rendering architecture with the eventual aim of reducing code duplication.

A key design goal is the decoupling of graphics API specifics from the
core rendering logic. It introduces an API context interface
(libmpv_gpu_next.h) that separates the API-specific host duties (like
wrapping a user-provided framebuffer) from the shared, API-agnostic
video engine (video.c).

This initial implementation provides an OpenGL backend, which is selected
when the client specifies MPV_RENDER_API_TYPE_OPENGL. The architecture is
designed to easily accommodate other backends like Vulkan or D3D11 in
the future.

To use it, libmpv clients can pass the MPV_RENDER_PARAM_BACKEND
parameter with the value "gpu-next" during the call to
mpv_render_context_create.

Resolves:
#15107
#10810

[sparky3387]

For testing purposes, I have also got a fork with the changes needed for Haruna/mpvQt here:
https://github.com/sparky3387/mpvqt

[kasper93]

Thanks, I will take a look when I get a longer second.

/cc @haasn if you are interested.

[github-actions]

Download the artifacts for this pull request:

Windows

• mpv-aarch64-pc-windows-msvc
• mpv-i686-w64-mingw32
• mpv-x86_64-pc-windows-msvc
• mpv-x86_64-w64-mingw32

macOS

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

[sparky3387]

For anyone out there on nix os patch can be used with:

let
  # 1. Define your custom mpv-unwrapped from your fork.
  # We override pkgs.mpv-unwrapped, as that is the actual dependency.
  mpv-unwrapped-from-fork = pkgs.mpv-unwrapped.overrideAttrs (oldAttrs: rec {
    pname = "mpv-unwrapped-from-fork";
    version = "git-unstable";
    src = pkgs.fetchFromGitHub {
      owner = "sparky3387";
      repo = "mpv";
      rev = "master";
      hash = "sha256-LAicqT7R832vIFULi+cyDi+XHKNK+03p8ncf5GHVEh4=";	  
    };
  });

  # 2. Define your custom mpvqt using overrideAttrs.
  # This correctly modifies the `extraPropagatedBuildInputs` list.
  mpvqt-from-fork = pkgs.kdePackages.mpvqt.overrideAttrs (oldAttrs: {
    pname = "mpvqt-from-fork";
    version = "git-unstable";

    # Change the source to our fork
    src = pkgs.fetchFromGitHub {
      owner = "sparky3387";
      repo = "mpvqt";
      rev = "master";
      hash = "sha256-nrKN+grQZou2KNqi2SnjkGsudq5CHyioGixHHGIySNc=";
    };

    # We remove the original `mpv-unwrapped` from the correct list
    # and add our new version from your fork.
    propagatedBuildInputs = lib.lists.remove pkgs.mpv-unwrapped oldAttrs.propagatedBuildInputs ++ [ mpv-unwrapped-from-fork ];
  });

  # 3. Override haruna to use your custom mpvqt.
  # This part has been correct.
  haruna-custom = pkgs.haruna.override {
    kdePackages = pkgs.kdePackages // { mpvqt = mpvqt-from-fork; };
  };

in
{
  home.packages = with pkgs; [
haruna-custom
};