FlexASIO should not open the input or output stream direction if it's not used

[dechamps]

Currently, if both input and output devices are available, but the ASIO Host Application only opens channels for one direction (i.e. input or output), FlexASIO will still open a full-duplex PortAudio stream. In other words, FlexASIO calls Pa_OpenStream() with both input and output devices even if the ASIO Host Application calls createBuffers() with a set of ASIOBufferInfos that are all input channels, or all output channels.

Ideally, FlexASIO should not open a device that it's not going to use, because that increases the likelihood of clock mismatch issues.

As part of #22 I will introduce a way to disable one side of the stream. This helps, but ideally FlexASIO should just figure this out on its own.

[dechamps]

Note that this enhancement, on its own, will not help with audio format negotiation, because FlexASIO will still attempt to open a stream in canSampleRate(), but at this point FlexASIO doesn't know which direction will be used yet. This probably means FlexASIO should stop trying to open a stream in canSampleRate() and instead should act as if any sample rate is supported; this would probably result in a net improvement to user experience, especially when using WASAPI Shared (see #23).

[dechamps]

Actually, I just thought of an even better way to solve the above problem: in canSampleRate(), try to open two streams: an input-only stream and an output-only stream. Return false only if both streams fail to open. This will still allow canSampleRate() to return mostly accurate information, in the sense that it will still accept a sample rate if there is any way to make it work.