mixausrc: fix ausrc resampling

[cspiel1]

• Fixes resampling of audio source which is mixed into the stream.
• Does some code cleanup.
• Adds warning if sample format is not supported.

Because auresamp filter can be loaded at arbitrary position, the sample rate has to be taken from the stream in functions encode()/decode().

mentioned here: #2938 (comment)

[cspiel1]

Tested mixing into

• the encode stream
• the decode stream
  with module aufile and WAV files
• Microsoft PCM, 16 bit, mono 8000 Hz
• Microsoft PCM, 16 bit, mono 16000 Hz
• Microsoft PCM, 16 bit, mono 32000 Hz
  which works.

The following formats bring a useful warning and stops mixing

• ITU G.711 mu-law -> "mixausrc: sample format PCMU not supported"
• Microsoft PCM, 16 bit, mono 44100 Hz -> "mixausrc: could not initialize a resampler (Operation not supported [95])"

Ready for merge from my side. Any suggestions?