Make it easier to get PortAudio debug logs

[dechamps]

Some issues can be difficult to troubleshoot even from the FlexASIO debug logs, because often the issue is coming from somewhere within PortAudio. FlexASIO does surface PortAudio error codes in its debug logs but these are notoriously vague, e.g. paInvalidDevice can mean any number of things (see #3 for an example).

PortAudio itself has fairly detailed error logs (see pa_debugprint.c), but they're not easy to get to. They get logged to STDERR, which is accessible for console applications (such as jackd), but, AFAIK, not for graphical applications. As a result, in most cases there is no way to get them.

Fortunately, PortAudio seems to provide a function, PaUtil_SetDebugPrintFunction(), that allows library users to override this behaviour and intercept debug messages themselves. FlexASIO could use that to incorporate PortAudio debug messages into its own debug log.