OpenDevice was a confusing name because it does not actually open a
device.

Postpone the Start() call until there is something to be played.

This finishes problems which occur early in the WasapiOutputThread;
previously, the error was ignored and the output blocked forever
without doing anything (and without reporting the error).

This is duplicate; this has already been done in Close().

This is pointless; the method cannot be called again anyway.

It is pointless to let WasapiOutputThread wait for the
CheckException() call.

If an exception has been caught, the method cannot continue playback,
therefore it doesn't make sense to have the "catch" block inside the
"while" block (and not break the loop after catching an exception).

The current Drain() implementation does what Cancel() should do; it
does not wait for completion, but instead discards the buffer.

Fixes those programs on Windows.

MSDN documentation suggests always passing this flag to reduce
overhead for an "obsolete technology".

None of the current FormatHResultError() callers need the format string.

Parameters should better be captured.  This removes some complexity
from Async().