Ogg Opus updates from Ian Nartowicz.

These are the changes proposed in
 <http://www.ietf.org/mail-archive/web/codec/current/msg03058.html>,
 with some minor additions (see follow-up on the list).

doc/draft-ietf-codec-oggopus.xml

@@ -688,9 +688,9 @@ sample *= pow(10, output_gain/(20.0*256)) ,
 <vspace blankLines="1"/>
 Virtually all players and media frameworks should apply it by default.
 If a player chooses to apply any volume adjustment or gain modification, such
- as the R128_TRACK_GAIN, R128_ALBUM_GAIN (see <xref target="comment_header"/>)
- or a user-facing volume knob, the adjustment MUST be applied in addition
- to this output gain in order to achieve playback at the desired volume.
+ as the R128_TRACK_GAIN (see <xref target="comment_header"/>), the adjustment
+ MUST be applied in addition to this output gain in order to achieve playback
+ at the normalized volume.
 <vspace blankLines="1"/>
 An encoder SHOULD set this field to zero, and instead apply any gain prior to
  encoding, when this is possible and does not conflict with the user's wishes.
@@ -1215,15 +1215,10 @@ If present, R128_TRACK_GAIN and R128_ALBUM_GAIN MUST correctly represent
 If a player chooses to make use of the R128_TRACK_GAIN tag or the
  R128_ALBUM_GAIN tag, it MUST apply those gains
  <spanx style="emph">in addition</spanx> to the 'output gain' value.
-</t>
-<t>
-If an encoder wishes to use R128 normalization, and the output gain is not
- otherwise constrained or specified, the encoder SHOULD write the R128 gain
- into the 'output gain' field and store a tag containing "R128_TRACK_GAIN=0".
-That is, it should assume that by default tools will respect the 'output gain'
- field, and not the comment tag.
 If a tool modifies the ID header's 'output gain' field, it MUST also update or
  remove the R128_TRACK_GAIN and R128_ALBUM_GAIN comment tags if present.
+An encoder should assume that by default tools will respect the 'output gain'
+ field, and not the comment tag.
 </t>
 <t>
 To avoid confusion with multiple normalization schemes, an Opus comment header