Fix cache size configuration

- changes to the correct mpv option "demuxer-max-bytes"
- fix size calcuation. the configurable cache values of [10, 75, 150,
  500] are presumed to be megabytes

src/player/PlayerComponent.cpp

@@ -1266,7 +1266,7 @@ void PlayerComponent::updateVideoSettings()
   setAudioDelay(m_playbackAudioDelay);
 
   QVariant cache = SettingsComponent::Get().value(SETTINGS_SECTION_VIDEO, "cache");
-  mpv::qt::set_property(m_mpv, "cache", cache.toInt() * 1024);
+  mpv::qt::set_property(m_mpv, "demuxer-max-bytes", cache.toInt() * 1024 * 1024);
 
   updateVideoAspectSettings();
 }