CLI: make --mirror available in standard non-batch mode via

new routines `mirror_start()`, `mirror_outfile()` & limit
  substitutions on Unix to backslash (Windows remains the same);
  restrict "borderwidth", "textgap", "vwhitesp", "whitesp" < 100
CODEONE: fix `ceilf()` -> `C1_MULT_CEIL()` in FAST_MODE encodation
  (improves/changes some encodation)
composite: preserve `gs1_verify()` warning using `warn_level` hack;
  fix wrong name `zint_dbar_omnstk_set_height()` ->
  `zint_dbar_stk_set_height()`;
  use new `zint_micropdf_variant()` & `zint_micropdf_expand()`
  routines (see below)
MICROPDF417: move variant determination into new shareable routine
  `zint_micropdf_variant()` & expansion into `zint_micropdf_expand()`
qzint: add helper methods `setbordertypevalue()`,
  `encodedinputmode()`, `encodedoutputoptions()`,
  `ecivaluetoeciindex()` and `eciindextoecivalue()`
dbar: for performance pre-calculate `combins()` as array
coverage: add `malloc()`-type failure testing to AZTEC,
  CONTENT_SEGS, MEMORY_FILE, raster, Reed-Solomon & vector;
  numerous changes to remove unused code and test more branches
common: add `isxdigit()`
general: various code fiddlings
manual: add char names to sequence and batch format char tables;
  mention long options can be shortened (& use "--compliantheight"
  -> "--compliant");
  expand mirror explanation & document substitutions in footnote;
  workaround xecjk putting space after double quote when quoting
  cjk chars & rejig input mode examples;
  workaround html alignment bug in table gs1-enabled symbologies by
  adding extra rh dashes to header cols;
  add "figure nn: " to txt image tags via sed:
  various other small fixes and rephrasings

backend/output.c

@@ -189,7 +189,7 @@ INTERNAL int zint_out_colour_get_cmyk(const char *colour, int *cyan, int *magent
     return 1 + have_alpha;
 }
 
-/* Convert internal colour chars "WCBMRYGK" to RGB */
+/* Convert internal colour chars "WCBMRYGK" to RGB. Returns 1 on success, else 0 */
 INTERNAL int zint_out_colour_char_to_rgb(const unsigned char ch, unsigned char *red, unsigned char *green,
                 unsigned char *blue) {
     static const char chars[] = "WCBMRYGK";
@@ -955,6 +955,12 @@ static int out_maybe_mkdir(const char *path) {
     return 0;
 }
 
+#ifdef ZINT_TEST /* Wrapper for direct testing */
+INTERNAL int zint_test_out_maybe_mkdir(const char *path) {
+    return out_maybe_mkdir(path);
+}
+#endif
+
 /* Create output file, creating sub-directories if necessary. Returns `fopen()` FILE pointer */
 INTERNAL FILE *zint_out_fopen(char filename[256], const char *mode) {
     FILE *outfile;