Replace a couple more #defines with inline functions.

My trawl of all the vtable systems in the code spotted a couple of
other function-like macros in passing, which might as well be
rewritten as inline functions too for the same reasons.

windows/winstuff.h

@@ -41,7 +41,11 @@
 struct Filename {
     char *path;
 };
-#define f_open(filename, mode, isprivate) ( fopen((filename)->path, (mode)) )
+static inline FILE *f_open(const Filename *filename, const char *mode,
+                           bool isprivate)
+{
+    return fopen(filename->path, mode);
+}
 
 struct FontSpec {
     char *name;