[BUGFIX] Fix standalone app's audio input

[sdatkinson]

Description

• Allow choosing audio inputs other than the first one (Scarlett Solo can use its 1/4" input!)
• Support for single-input devices maintained.

Resolves #157.
Resolves #209.

PR Checklist

• Did you format your code using format.bash?
• Does the VST3 plugin pass all of the unit tests in the VST3PluginTestHost? (Download it as part of the VST3 SDK here.)
  • Windows
  • macOS
• [N] Does your PR add, remove, or rename any plugin parameters? If yes...
  • [N/A] Have you ensured that the plug-in unserializes correctly?
  • [N/A] Have you ensured that older versions of the plug-in load correctly? (See Unserialization.cpp.)
• [N] Does your PR add or remove any graphical assets? If yes, are they defined in config.h and added in the two required locations in main.rc?

Additional checks:

• Works on macOS with Scarlett 2i2 both inputs and with the microphone input (single-input example)
• Works on Windows 11 with Scarlett 18i20 Gen 1 with inputs 1, 2, and 3.

[sdatkinson]

Finally! 🎉