fix(mpv): auto-select subtitle and audio tracks on file open (#12)

Closes #7

mpv only turns on a subtitle that is flagged default or matches `slang`, so releases like VisionQuest (Russian dub by default, ru/ru/en subtitles, none flagged) open with subtitles off, which also disabled **Translate subtitle**.

- New `TrackSelection` helpers in `qtpotato_core` with unit tests.
- On file load: audio follows *Preferred languages* (Playback settings); subtitle follows *Preferred languages* (Subtitles settings), else keeps mpv's choice, else (new *Turn on a subtitle when none matches*, on by default) picks a full text subtitle in the system language, then English, then the audio language, then the default/first track.
- Translate is enabled whenever the file has a text subtitle; with none or a bitmap subtitle shown it translates the best text track (English first).

Verified in the real app on VisionQuest S01E01: subtitle 3 (en) is selected on open; with audio set to `en` and auto-select off, audio switches to track 7, subtitle stays off and Translate stays enabled with the English source.

Reviewed-on: #12
Co-authored-by: Tri Vo Khac <git@trivk.dev>

src/mpv/trackselection.h

@@ -0,0 +1,41 @@
+#pragma once
+
+#include <QList>
+#include <QString>
+#include <QStringList>
+
+// Which audio and subtitle tracks to turn on when a file opens. mpv only turns on a
+// subtitle that is flagged default or matches --slang, so many files open with none.
+namespace TrackSelection
+{
+struct Track {
+    int id = 0;
+    QString lang;
+    QString codec;
+    bool forced = false;
+    bool isDefault = false;
+};
+
+// True for picture-based subtitles (PGS, VobSub, DVB), which can't be translated.
+// \a codec is the FFmpeg codec name that mpv reports.
+bool isBitmapSubtitle(const QString &codec);
+
+// Compares language tags written as ISO 639-1/2 codes ("en", "eng"), with a region
+// ("en-US") or as English names ("English").
+bool sameLanguage(const QString &a, const QString &b);
+
+// The first audio track in a \a preferredLanguages language, else \a currentId.
+int pickAudio(const QList<Track> &tracks, const QStringList &preferredLanguages, int currentId);
+
+// The subtitle to turn on when the file opens. A track in one of \a preferredLanguages
+// wins over mpv's own choice \a currentId; without one, \a currentId is kept. When mpv
+// chose nothing and \a fallback is set, picks a full (not forced) subtitle in the first
+// of \a fallbackLanguages that has one, then one flagged default, then the first one.
+// Text subtitles win over bitmap ones in the same step. Returns 0 for no subtitle.
+int pickSubtitle(const QList<Track> &tracks, const QStringList &preferredLanguages, const QStringList &fallbackLanguages, int currentId, bool fallback);
+
+// The subtitle to translate: \a currentId when it is a text subtitle, else a full text
+// subtitle in the first of \a sourceLanguages that has one, then one flagged default,
+// then the first one. 0 when there is no text subtitle.
+int pickTranslationSource(const QList<Track> &tracks, const QStringList &sourceLanguages, int currentId);
+}