Apply restrictions to FLAC decoder_config similar to Opus `decoder_…

…config`.

  - The `decoder_config` is shared between substreams. Some values do not make sense to apply to all substreams.

index.bs

@@ -121,10 +121,16 @@ url: https://en.wikipedia.org/wiki/Q_(number_format); spec: Q-Format; type: dfn;
 
 url: https://xiph.org/flac/format.html; spec: FLAC; type: dfn;
 	text: METADATA_BLOCK
+	text: METADATA_BLOCK_STREAMINFO
 	text: FRAME
 	text: FRAME_HEADER
 	text: SUBFRAME
 	text: FRAME_FOOTER
+	text: minimum block size
+	text: maximum block size
+	text: minimum frame size
+	text: maximum frame size
+	text: number of channels
 
 url: https://xiph.org/flac/format.html; spec: FLAC; type: property;
 	text: fLaC
@@ -1618,7 +1624,13 @@ The sample rate used for computing offsets SHALL be the rate indicated by the [=
 
 [=codec_id=] SHALL be 'fLaC', the FLAC stream marker in ASCII, meaning byte 0 of the stream is 0x66, followed by 0x4C 0x61 0x43.
 
-[=decoder_config()=] for FLAC is [=METADATA_BLOCK=] of [[!FLAC]].
+[=decoder_config()=] for FLAC are the [=METADATA_BLOCK=]s of [[!FLAC]]. The [=METADATA_BLOCK_STREAMINFO=] has the following constraints:
+- [=minimum block size=] block SHALL be set to [=num_samples_per_frame=].
+- [=maximum block size=] block SHALL be set to [=num_samples_per_frame=].
+- [=minimum frame size=] block SHALL be set to 0.
+- [=maximum frame size=] block SHALL be set to 0.
+- [=number of channels=] block SHALL be set to 2. [=number of channels=] can be ignored because the real value can be determined from the [=Audio Element OBU=].
+
 
 The format of [=audio_frame()=] is [=FRAME=] of [[!FLAC]], which is composed of [=FRAME_HEADER=], followed by [=SUBFRAME=](s) (one [=SUBFRAME=] per channel) and followed by [=FRAME_FOOTER=].