PlayerComponent: add audio_delay.50hz setting

A new setting is not so great, but we probably can't get around it.

(Unless we find that applying the 25hz setting to 50hz is always ok.)

Fixes #521.

resources/settings/settings_description.json

@@ -296,6 +296,16 @@
           "step": 50
         }
       },
+      {
+        "value": "audio_delay.50hz",
+        "default": 0,
+        "hidden": true,
+        "possible_range": {
+          "min": -300,
+          "max": 300,
+          "step": 50
+        }
+      },
       {
         "value": "cache",
         "default": 75,

src/player/PlayerComponent.cpp

@@ -806,6 +806,8 @@ void PlayerComponent::setAudioDelay(qint64 milliseconds)
     audioDelaySetting = "audio_delay.24hz";
   else if (fabs(displayFps - 25) < 0.5)
     audioDelaySetting = "audio_delay.25hz";
+  else if (fabs(displayFps - 50) < 0.5)
+    audioDelaySetting = "audio_delay.50hz";
 
   double fixedDelay = SettingsComponent::Get().value(SETTINGS_SECTION_VIDEO, audioDelaySetting).toFloat();
   mpv::qt::set_property(m_mpv, "audio-delay", (fixedDelay + m_playbackAudioDelay) / 1000.0);