Fixing warnings and minor subtle typo.

This is derived from the ma_device_type_duplex case. It basically
detects a possible overrun and drops some periods. The idea is to
prevent the buffer from indefinitely straddling the end of the buffer
and causing persistent glitching.

Public issue https://github.com/dr-soft/miniaudio/issues/81

Public issue https://github.com/dr-soft/miniaudio/issues/146

This was causing warnings on some older Visual Studio compilers.

Public issue https://github.com/dr-soft/miniaudio/issues/145

Yet another compilation error with this setting enabled. Now removing
this entirely because I'm tired of maintaining it for almost no
practical benefit.

Mark some non-static functions as MA_API