Document that the order of cues passed to TextOutput is important

Fix WebViewSubtitleOutput to respect this (CanvasSubtitleOutput already
does).

Issue: #8704
PiperOrigin-RevId: 365534018

library/common/src/main/java/com/google/android/exoplayer2/text/TextOutput.java

@@ -25,6 +25,9 @@ public interface TextOutput {
   /**
    * Called when there is a change in the {@link Cue Cues}.
    *
+   * <p>{@code cues} is in ascending order of priority. If any of the cue boxes overlap when
+   * displayed, the {@link Cue} nearer the end of the list should be shown on top.
+   *
    * @param cues The {@link Cue Cues}. May be empty.
    */
   void onCues(List<Cue> cues);

library/ui/src/main/java/com/google/android/exoplayer2/ui/WebViewSubtitleOutput.java

@@ -278,6 +278,7 @@ import java.util.Map;
               Util.formatInvariant(
                   "<div style='"
                       + "position:absolute;"
+                      + "z-index:%s;"
                       + "%s:%.2f%%;"
                       + "%s:%s;"
                       + "%s:%s;"
@@ -287,6 +288,7 @@ import java.util.Map;
                       + "background-color:%s;"
                       + "transform:translate(%s%%,%s%%);"
                       + "'>",
+                  /* z-index */ i,
                   positionProperty,
                   positionPercent,
                   lineProperty,