mpv: fix issue #1725 and add tests (#1726)

Closes issue #1725.

This allows mpv module to be customized with support for more advanced
features than the `programs.mpv.scripts` current support. For example,
with this change now this is possible:

```nix
{
  programs.mpv.package = (pkgs.wrapMpv (pkgs.mpv-unwrapped.override {
    vapoursynthSupport = true;
  }) {
    extraMakeWrapperArgs = [
      "--prefix" "LD_LIBRARY_PATH" ":" "${pkgs.vapoursynth-mvtools}/lib/vapoursynth"
    ];
  });
}

```

Since `programs.mpv.package` doesn't necessary reflect the final
derivation anymore (see #1524), we introduce `programs.mpv.finalPackage`
that has the resulting derivation.

This includes 2 tests:
- One to check if everything is alright with mpv
- Other to validate our assertion that package and scripts can't be
  passed both at the same time

* docs: document recent mpv module changes

* mpv: add thiagokokada as maintainer

modules/programs/mpv.nix

@@ -50,7 +50,7 @@ let
     renderOptions { profile = concatStringsSep "," profiles; };
 
   mpvPackage = if cfg.scripts == [ ] then
-    pkgs.mpv
+    cfg.package
   else
     pkgs.wrapMpv pkgs.mpv-unwrapped { scripts = cfg.scripts; };
 
@@ -60,8 +60,19 @@ in {
       enable = mkEnableOption "mpv";
 
       package = mkOption {
+        type = types.package;
+        default = pkgs.mpv;
+        example = literalExample
+          "pkgs.wrapMpv (pkgs.mpv-unwrapped.override { vapoursynthSupport = true; }) { youtubeSupport = true; }";
+        description = ''
+          Package providing mpv.
+        '';
+      };
+
+      finalPackage = mkOption {
         type = types.package;
         readOnly = true;
+        visible = false;
         description = ''
           Resulting mpv package.
         '';
@@ -91,7 +102,7 @@ in {
         example = literalExample ''
           {
             profile = "gpu-hq";
-            force-window = "yes";
+            force-window = true;
             ytdl-format = "bestvideo+bestaudio";
             cache-default = 4000000;
           }
@@ -153,9 +164,16 @@ in {
   };
 
   config = mkIf cfg.enable (mkMerge [
+    {
+      assertions = [{
+        assertion = (cfg.scripts == [ ]) || (cfg.package == pkgs.mpv);
+        message = ''
+          The programs.mpv "package" option is mutually exclusive with "scripts" option.'';
+      }];
+    }
     {
       home.packages = [ mpvPackage ];
-      programs.mpv.package = mpvPackage;
+      programs.mpv.finalPackage = mpvPackage;
     }
     (mkIf (cfg.config != { } || cfg.profiles != { }) {
       xdg.configFile."mpv/mpv.conf".text = ''
@@ -170,5 +188,5 @@ in {
     })
   ]);
 
-  meta.maintainers = with maintainers; [ tadeokondrak ];
+  meta.maintainers = with maintainers; [ tadeokondrak thiagokokada ];
 }