CEA-608: Don't assume roll-up captions are at the bottom of the screen

ANSI/CTA-608-E R-2014 Annex B.5 says:
"The concept of a movable base row for a roll-up caption is new."

It means "new" compared to TC1 and TC2 (released in or before 1985).

Issue: #7475
PiperOrigin-RevId: 315258859

RELEASENOTES.md

@@ -122,6 +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)).
 *   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

@@ -947,7 +947,7 @@ public final class Cea608Decoder extends CeaDecoder {
       int lineAnchor;
       int line;
       // Note: Row indices are in the range [1-15].
-      if (captionMode == CC_MODE_ROLL_UP || row > (BASE_ROW / 2)) {
+      if (row > (BASE_ROW / 2)) {
         lineAnchor = Cue.ANCHOR_TYPE_END;
         line = row - BASE_ROW;
         // Two line adjustments. The first is because line indices from the bottom of the window