Support latency

[bradkeifer]

Implement support for setting the latency value via the cli argument

[bradkeifer]

Check struct player_source in player.c. Perhaps the output_buffer_samples element is somewhat equivalent to our --latency command line argument. It is hardcoded presently.

In airplay.c we have:

rms->output_buffer_samples = OUTPUTS_BUFFER_DURATION * quality->sample_rate;

In outputs.h we have:

// Number of seconds the outputs should buffer before starting playback. Note
// this value cannot freely be changed because 1) some Airplay devices ignore
// the values we give and stick to 2 seconds, 2) those devices that can handle
// different values can only do so within a limited range (maybe max 3 secs)
#define OUTPUTS_BUFFER_DURATION 2

Given the comment in outputs.h, perhaps we are best to stick with a hardcoded solution here. To be discussed with @marcelveldt