(Try to) fix warnings that popped up in #4223

The only place I don't know how to fix is the
`-fvar-tracking-assignments` warning in `ProcParams::save()` as
there is nothing to constify.

rtexif/rtexif.cc

@@ -1671,7 +1671,7 @@ void Tag::toString (char* buffer, int ofs)
 
     strcpy (buffer, "");
 
-    for (size_t i = 0; i < std::min<int>(maxcount, valuesize - ofs); i++) {
+    for (ssize_t i = 0; i < std::min<int>(maxcount, valuesize - ofs); i++) {
         if (i > 0) {
             strcat (buffer, ", ");
         }