CEA-608: Position top-of-screen roll-up cues with bottom-line=row

Reasoning and screenshots here:
#7475 (comment)

Issue: #7475
PiperOrigin-RevId: 315475888

RELEASENOTES.md

@@ -122,8 +122,8 @@
     *   Ignore excess characters in CEA-608 lines (max length is 32)
         ([#7341](https://github.com/google/ExoPlayer/issues/7341)).
     *   Add support for WebVTT's `ruby-position` CSS property.
-    *   Don't assume CEA-608 captions are in the bottom half of the
-        screen([#7475](https://github.com/google/ExoPlayer/issues/7475)).
+    *   Fix positioning for CEA-608 roll-up captions in the top half of screen
+        ([#7475](https://github.com/google/ExoPlayer/issues/7475)).
 *   DRM:
     *   Add support for attaching DRM sessions to clear content in the demo app.
     *   Remove `DrmSessionManager` references from all renderers.

library/core/src/main/java/com/google/android/exoplayer2/text/cea/Cea608Decoder.java

@@ -946,7 +946,7 @@ public Cue build(@Cue.AnchorType int forcedPositionAnchor) {
 
       int lineAnchor;
       int line;
-      // Note: Row indices are in the range [1-15].
+      // Note: Row indices are in the range [1-15], Cue.line counts from 0 (top) and -1 (bottom).
       if (row > (BASE_ROW / 2)) {
         lineAnchor = Cue.ANCHOR_TYPE_END;
         line = row - BASE_ROW;
@@ -955,9 +955,12 @@ public Cue build(@Cue.AnchorType int forcedPositionAnchor) {
         line -= 2;
       } else {
         lineAnchor = Cue.ANCHOR_TYPE_START;
-        // Line indices from the top of the window start from 0, but we want a blank row to act as
-        // the safe area. As a result no adjustment is necessary.
-        line = row;
+        // The `row` of roll-up cues positions the bottom line (even for cues shown in the top
+        // half of the screen), so we need to consider the number of rows in this cue. In
+        // non-roll-up, we don't need any further adjustments because we leave the first line
+        // (cue.line=0) blank to act as the safe area, so positioning row=1 at Cue.line=1 is
+        // correct.
+        line = captionMode == CC_MODE_ROLL_UP ? row - (captionRowCount - 1) : row;
       }
 
       return new Cue(