This is an internal change to unify and simplify the handling of file
paths.
On the Windows platform, the file path is always converted to UTF-8 and
converted to UTF-16 only for calls to WinAPI functions.
The behavior of the functions for opening files on non-Windows platforms
does not change.
Use standard `_WIN32` define instead. Any sane compiler on Windows
platfrom defines `_WIN32`.
The `wpath` parameter type has been changed to an equivalent type that
does not require the inclusion of the `windows.h` header.
* Use the correct scale when normalising an integer read on the fallback floating point path.
Without this the values were only being scaled to a short's max value.
* Add a second pass over the floating point tests that enables the replacement read functionality.
These cover the issue fixed by the previous commit.
* Add an entry to CHANGELOG.md for #702.
