Replace glib format conversion specifiers with inttypes ones

On Mac OS X, int64_t and gint64 are not the same type.  Since we now
require C99, we can freely use inttypes.h.

The standard mandates that inttypes.h include stdint.h.  Include
inttypes.h from openslide-private.h, since it is likely to be needed
most places for debugging, and drop stdint.h elsewhere in the library.
Outside the library, replace stdint.h includes with inttypes.h.