Setting for subtitle margin & fix subtitle opacity settings (#338)

Adds a subtitle style option to set the margin below unstyled subtitles.
This is roughly the percent of the screen below the subtitles. It
applies to both ExoPlayer & MPV.

Also fixes font & background opacity not being applied in some cases.

Fixes #225
Closes #178

app/src/main/java/com/github/damontecres/wholphin/preferences/AppPreferencesSerializer.kt

@@ -152,4 +152,5 @@ fun SubtitlePreferences.Builder.resetSubtitles() {
     backgroundColor = SubtitleSettings.BackgroundColor.defaultValue.toArgb()
     backgroundOpacity = SubtitleSettings.BackgroundOpacity.defaultValue.toInt()
     backgroundStyle = SubtitleSettings.BackgroundStylePref.defaultValue
+    margin = SubtitleSettings.Margin.defaultValue.toInt()
 }