Shaders: Anime4K presets in-session and in Settings

Vendored Anime4K v4.0.1 GLSL (MIT), extracted to app support once per
version. Presets A/B/C/A+A/B+B/C+A in HQ and Fast per Anime4K's mpv
templates, plus custom files. Shader button in the video controls
(works in fullscreen), Ctrl+1..6 / Ctrl+0, live apply with read-back,
failures fall back to Off with a toast. Render mode detected from
mpv's GL log; warns when on the software path. Docs include the
Jellyfin client section.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/desktop/lib/src/shaders/shader_assets.dart

@@ -0,0 +1,74 @@
+import 'dart:io';
+
+import 'package:flutter/services.dart';
+import 'package:path_provider/path_provider.dart';
+
+import 'shader_presets.dart';
+
+const anime4kAssetDir = 'assets/shaders/anime4k';
+
+/// mpv needs real files: copies the shipped Anime4K shaders out of the
+/// asset bundle into `<supportDir>/shaders/anime4k-<version>`, once per
+/// version (a `.complete` marker is written last, so a cut-short copy is
+/// redone). Returns that directory.
+Future<Directory> extractAnime4k(
+  Directory supportDir, {
+  AssetBundle? bundle,
+}) async {
+  final dir = Directory(
+      '${supportDir.path}${Platform.pathSeparator}shaders'
+      '${Platform.pathSeparator}anime4k-$anime4kVersion');
+  final marker = File('${dir.path}${Platform.pathSeparator}.complete');
+  final files = anime4kAllFiles;
+  if (await marker.exists() &&
+      files.every((f) => File(_join(dir, f)).existsSync())) {
+    return dir;
+  }
+  await dir.create(recursive: true);
+  final b = bundle ?? rootBundle;
+  for (final name in [...files, 'LICENSE']) {
+    final data = await b.load('$anime4kAssetDir/$name');
+    await File(_join(dir, name)).writeAsBytes(
+        data.buffer.asUint8List(data.offsetInBytes, data.lengthInBytes),
+        flush: true);
+  }
+  await marker.writeAsString(anime4kVersion, flush: true);
+  return dir;
+}
+
+/// Absolute paths of [preset]'s Anime4K files inside [dir].
+List<String> anime4kPaths(
+        Directory dir, ShaderPreset preset, ShaderQuality quality) =>
+    [for (final f in anime4kFiles(preset, quality)) _join(dir, f)];
+
+String _join(Directory dir, String name) =>
+    '${dir.path}${Platform.pathSeparator}$name';
+
+/// Where the shipped shaders get extracted; swapped in tests.
+Future<Directory> Function() shaderSupportDir = getApplicationSupportDirectory;
+
+/// Absolute `glsl-shaders` entries for a choice, extracting the shipped
+/// files first when needed. Empty for [ShaderPreset.off].
+Future<List<String>> shaderPathsFor(
+  ShaderPreset preset,
+  ShaderQuality quality,
+  List<String> custom,
+) async {
+  if (preset == ShaderPreset.off) return const [];
+  if (preset == ShaderPreset.custom) return [...custom];
+  final dir = await extractAnime4k(await shaderSupportDir());
+  return anime4kPaths(dir, preset, quality);
+}
+
+/// Why [paths] can't go to mpv (a missing file, or a path holding the
+/// list separator), or null when they can.
+String? shaderPathsProblem(List<String> paths, {bool? windows}) {
+  final sep = shaderListSeparator(windows: windows);
+  for (final p in paths) {
+    final name = p.split(RegExp(r'[/\\]')).last;
+    // Windows drive letters ("C:\") are fine: there the separator is ';'.
+    if (p.contains(sep)) return '"$name" has "$sep" in its path';
+    if (!File(p).existsSync()) return '"$name" not found';
+  }
+  return null;
+}