[mythtv] ReplayGain support for MythMusic

[Chris Hamilton]

I've created a ticket and attached a patch adding a software preamp to
AudioOutput and AudioOutputBase.  I tested it on my system by adding
some code to MythMusic that randomly changed the gain each time play()
was called, and it appears to work smoothly.  Adding this is quite low
risk because the preamp is disabled by default, and must be specifically
initialized and enabled before that code path is executed.

http://svn.mythtv.org/trac/ticket/4804

A sanity check and maybe a few other people testing the code would be
appreciated.  Also, it currently only supports full 8-bit, 16-bit or
32-bit signed audio samples.  Will anything else ever be used?

I have some follow-up patches that implement necessary database changes
and metadata extraction.  Should I bother to submit this stuff, or
should I wait until the metadata extraction has been refactored to use
libtag exclusively?

Cheers,

Chris Hamilton