CLI: fix bug in "--scalexdimdp" in converting from "in" to "mm"

(was dividing instead of multiplying doh);
  make "--scalexdimdp" units check error messages better;
  add more "--test" tests & have `validate_units()` etc take
  `errbuf` arg to make them more test-friendly
GUI: `copy_to_clipboard()` unshadow `data` -> `fdata`
test suite: make `utf8_to_wide()` same as "backend/output.c" & fix
  return vals;
  add `QZint::save_to_memfile()` test

backend/output.c

@@ -892,6 +892,7 @@ INTERNAL float zint_out_large_bar_height(struct zint_symbol *symbol, const int s
 
 #ifdef _WIN32
 /* Convert UTF-8 to Windows wide chars. Ticket #288, props Marcel */
+/* Note if change this, change versions in "frontend/main.c" and "backend/tests/testcommon.c" also */
 #define utf8_to_wide(u, w, r) \
     { \
         int lenW; /* Includes terminating NUL */ \