libass: enable CoreText backend on darwin

libass has multiple font rendering backends which are autodetected by
the configure script.  On macOS, there is a CoreText backend, but it
was disabled in Nix's build because the relevant native frameworks
weren't visible to the configure script.  Fix that.

nixos/doc/manual/release-notes/rl-2311.section.md

@@ -519,6 +519,8 @@ The module update takes care of the new config syntax and the data itself (user
 
 - The Home Assistant module now offers support for installing custom components and lovelace modules. Available at [`services.home-assistant.customComponents`](#opt-services.home-assistant.customComponents) and [`services.home-assistant.customLovelaceModules`](#opt-services.home-assistant.customLovelaceModules).
 
+- `libass` now uses the native CoreText backend on Darwin, which may fix subtitle rendering issues with `mpv`, `ffmpeg`, etc.
+
 ## Nixpkgs internals {#sec-release-23.11-nixpkgs-internals}
 
 - The use of `sourceRoot = "source";`, `sourceRoot = "source/subdir";`, and similar lines in package derivations using the default `unpackPhase` is deprecated as it requires `unpackPhase` to always produce a directory named "source". Use `sourceRoot = src.name`, `sourceRoot = "${src.name}/subdir";`, or `setSourceRoot = "sourceRoot=$(echo */subdir)";` or similar instead.

pkgs/development/libraries/libass/default.nix

@@ -4,6 +4,7 @@
 , rasterizerSupport ? false # Internal rasterizer
 , largeTilesSupport ? false # Use larger tiles in the rasterizer
 , libiconv
+, darwin
 }:
 
 assert fontconfigSupport -> fontconfig != null;
@@ -29,7 +30,12 @@ stdenv.mkDerivation rec {
 
   buildInputs = [ freetype fribidi harfbuzz ]
     ++ lib.optional fontconfigSupport fontconfig
-    ++ lib.optional stdenv.isDarwin libiconv;
+    ++ lib.optional stdenv.isDarwin [
+      libiconv
+      darwin.apple_sdk.frameworks.ApplicationServices
+      darwin.apple_sdk.frameworks.CoreFoundation
+      darwin.apple_sdk.frameworks.CoreText
+    ];
 
   meta = with lib; {
     description = "Portable ASS/SSA subtitle renderer";