wasapi: Add paWinWasapiAutoConvert flag, which sets AUDCLNT_STREAMFLAGS_AUTOCONVERTPC...

wasapi: Add paWinWasapiAutoConvert flag, which sets AUDCLNT_STREAMFLAGS_AUTOCONVERTPCM | AUDCLNT_STREAMFLAGS_SRC_DEFAULT_QUALITY for Shared mode streams.

AUDCLNT_STREAMFLAGS_AUTOCONVERTPCM is required in order to allow for sample rates other than the system mixer configured sample rate. Otherwise, WASAPI is limited to a single system configured sample rate in Shared mode, which is a severe limitation compared to other APIs supported by PortAudio.
AUDCLNT_STREAMFLAGS_SRC_DEFAULT_QUALITY is recommended to be used additionally.
See <https://docs.microsoft.com/en-us/windows/desktop/coreaudio/audclnt-streamflags-xxx-constants> for documentation.

GetClosestFormat is modified to exit early when paWinWasapiAutoConvert is set in Shared mode because we trust the WASAPI conversions to succeed.

We restrict these changes to Windows 7 (and later) because documentation does not clearly state the minimum required Windows version. Windows Vista was not available for testing.

This patch has been in production in OpenMPT (<https://openmpt.org/>) since version 1.27.01.00 (2017-09-27). PortAudio WASAPI Shared mode is the default output device type in OpenMPT for Windows 7 (and later) and also for Wine setups.