Fix usage of ParsableByteArray in `PgsDecoder`

This is a partial revert of
https://github.com/google/ExoPlayer/commit/46598a46fd3c13a9be906abd6c673b9487ba5d50

The change from reset(int) to setPosition/Limit() in this file was
incorrect, the reset(int) call is important because it ensures
`bitmapData` is large enough for the `buffer.readBytes` call on L188.

Issue: #8417
PiperOrigin-RevId: 351110038

library/core/src/main/java/com/google/android/exoplayer2/text/pgs/PgsDecoder.java

@@ -177,8 +177,7 @@ public final class PgsDecoder extends SimpleSubtitleDecoder {
         }
         bitmapWidth = buffer.readUnsignedShort();
         bitmapHeight = buffer.readUnsignedShort();
-        bitmapData.setPosition(0);
-        bitmapData.setLimit(totalLength - 4);
+        bitmapData.reset(totalLength - 4);
         sectionLength -= 7;
       }