audio: Add G.711 decoder

[onkrot]

https://en.wikipedia.org/wiki/G.711

Both a-law and mu-law implemented as lookup tables

[Croworbit]

is there content that this is used for?

[Dinnerbone]

(or at least a crafted example so we can test with, please!)
Thank you!

[Lord-McSweeney]

I don't think it should be added to the AudioCompression struct in SWF, since it should (?) only appear in FLVs- is there another way to add it?

[lvyitian]

I don't think it should be added to the AudioCompression struct in SWF, since it should (?) only appear in FLVs- is there another way to add it?

@Lord-McSweeney Same in 409a713#diff-00d1b36fa29c1b47759997928f850c8ff051e7bcbe201ab073f0f855f1d3ecdeR1029
This has actually already been discussed in https://discord.com/channels/610531541889581066/614855396468719619/1244921689221894174

[torokati44]

At least I personally have changed my mind since then. Now I think it's fine. After all, we have also added H264 as VideoCodec...

[onkrot]

Okay, this definitely needed more testing (see #16614)

[onkrot]

FFmpeg commands for flv encoding
A-law: ffmpeg -i output.wav -acodec pcm_alaw -ar 8000 -ac 1 a.flv
Mu-law: ffmpeg -i output.wav -acodec pcm_mulaw -ar 8000 -ac 1 m.flv

[torokati44]

I'm still unable to make a passing test for this.

This is what FP plays the tone as:

And this is what this PR currently plays it as:

Maybe the FLV parser doesn't feed the decoder enough data...?

Could the same thing be causing the fairly common "MP3 decoder buffer underrun" errors?

I've also noticed that a decoder is being recreated for each "packet", which is probably not optimal...?

Pinging @kmeisthax.