SDL: Reduce audio playback latency

The previous default buffer size of 4096 samples for 44kHz mixer
would add up to 93ms of audio latency, which is fine for early
adventure games, but this is significantly more latency than is
acceptable for games with full motion video. For these games,
the latency needs to be kept within roughly +15ms and -45ms of
video frame presentation to avoid lip sync problems. With this
change, the default audio buffer size is calculated to be 1024
samples at 44kHz (which happens to match what DOSBox uses).

There is a possibility that the reduced latency may cause issues
that did not previously exist with things like the MT-32 emulator,
where a larger buffer size allowed for a larger window where
high-complexity synthesis that could not be generated in realtime
could be balanced out by low-complexity synthesis that could be
generated faster than realtime. In this case, rather than
increasing the system mixer buffer size again, please move the
MT-32 emulator into its own thread and give it its own larger ring
buffer into which it can generate more sample data in advance,
independently from the rest of the audio system.

For other systems where this buffer size reduction might cause a
problem with audio drop-outs, a new audio_buffer_size
configuration option has been added to allow users to tweak the
audio buffer size to match their machine's ability to generate
audio samples.

Fixes Trac#10033. Also improves playback of samples in SCI that
were programmed to restart across several consecutive frames,
relying on lower audio latency in the original engine for this to
not sound bad, like the hopping sound at the start of chapter 1
of KQ7, and the sound of turning on the power in the digger train
in the Lighthouse volcano.

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