Adds an "Anchor to Screen" toggle under Subtitle Styling (Android +
ExoPlayer only, default off). When enabled, the text SubtitleView is
sized to the full container instead of the letterboxed video rect, so
SRT/VTT/mov_text cues render in the black bars below widescreen video
and font size and the position setting become relative to the physical
screen height. Bitmap (PGS/VOB) and ASS/libass rendering are unchanged;
mpv already places plaintext subtitles in the margins by default.
close#1730
edde746
8e5279a487