Fix some VC6/2003 errors

This commit fixes an issue where the optimized mono expansion path is
never hit.

This is happening because the data buffer is incorrectly being reported
as busy (still loading) which is used to indicate to the engine that no
data is available and therefore nothing can be played.

This was always returning the input frame count rather than the number
of frames processed.

Public issue https://github.com/dr-soft/miniaudio/issues/191

VC6 doesn't appear to have SetFilePointerEX, so fallback on
SetFilePointer instead. This matches-up with VC6 not supporting
64-bit fseek.

Visual Studio .NET 2003 supports it, however.

For whatever reason, neither compiler likes implicit casts between
`volatile unsigned int*` and `int*`.

This is useful for enabling or disabling fading around loop
transitions.