feat(subtitles): optionally anchor text subtitles to the screen bottom

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

lib/i18n/ja.i18n.json

@@ -740,6 +740,8 @@
     "overrideStrip": "スタイルを削除",
     "positionTop": "上",
     "positionBottom": "下",
+    "anchorToScreen": "",
+    "anchorToScreenDescription": "",
     "bold": "太字",
     "italic": "斜体",
     "renderResolution": "レンダリング解像度",