Set a different style for HDR subtitles & adjust image subtitles opacity (#827)

## Description
Can now configure a different subtitle style for HDR playback.

There is also a new setting to change the opacity/dim image based
subtitles. This is primarily for HDR to dim very bright PGS subtitles,
but the setting is available for SDR too.

Please remember that the styles only apply to unstyled text formats like
SRT.

I haven't been able to consistently get the preview to switch the
display into HDR mode, so the preview is not 100% accurate.

### Related issues
Closes #583

### Testing
Shield pro 2019 w/ LG C2

## AI or LLM usage
None

app/src/main/proto/WholphinDataStore.proto

@@ -127,6 +127,7 @@ message SubtitlePreferences{
   bool font_italic = 10;
   int32 margin = 11;
   int32 edge_thickness = 12;
+  int32 image_subtitle_opacity = 13;
 }
 
 message LiveTvPreferences {
@@ -152,6 +153,7 @@ message InterfacePreferences {
   SubtitlePreferences subtitles_preferences = 7;
   LiveTvPreferences live_tv_preferences = 8;
   BackdropStyle backdrop_style = 9;
+  SubtitlePreferences hdr_subtitles_preferences = 10;
 }
 
 message AdvancedPreferences {