Win32: Only use large buffers when writing to disk

Windows can suffer quite badly from disk fragmentations. To avoid
this, on Windows, the FILE* buffer size was set to 10Meg. However,
this huge buffer is undesireable when writing to a eg a pipe.

This patch updates the behaviour to only use the huge buffer when
writing to disk.

Patch-from: lvqcl <lvqcl.mail@gmail.com>
Closes: https://sourceforge.net/p/flac/feature-requests/114/
1 file changed