flac encoder: enable Ogg FLAC and Ogg chaining

refactors GenerateOggSerial into a generic GenerateSerial
utility, under the util lib.

libFLAC may be encoded without Ogg support. If Ogg support is disabled,
libFLAC will still export Ogg-related methods (like setting a serial
number), and throw a runtime error when initializing an Ogg stream.

GenerateOggSerial does not depend on libogg. Refactoring it into
a generic GenerateSerial prevents having to add build-time checks
for libogg within the FLAC encoder plugin.

doc/plugins.rst

@@ -583,6 +583,10 @@ Encodes into `FLAC <https://xiph.org/flac/>`_ (lossless).
      - Description
    * - **compression**
      - Sets the libFLAC compression level. The levels range from 0 (fastest, least compression) to 8 (slowest, most compression).
+   * - **oggflac yes|no**
+     - Configures if the stream should be Ogg FLAC versus native FLAC. Defaults to "no" (use native FLAC).
+   * - **oggchaining yes|no**
+     - Configures if the stream should use Ogg Chaining for in-stream metadata. Defaults to "no". Setting this to "yes" also enables Ogg FLAC.
 
 lame
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