font-patcher: Fix patching of fonts with slashes in name

[why]
When trying to patch Anka/Coder fontforge reports it can not open the
font file.

[how]
When opening a font file we open all fonts within that file. For a ttf
or otf that is one font, but ttc can contain multiple fonts (of course).

The fontforge open function takes the font filename and the subfont name
in parenteses appended to the font filename. For example
`directory/folder/myfont.ttf` which contains just MyFont, it opens via
`directory/folder/myfont.ttf(MyFont)`.

That algorithm is not very smart, and so this fails
`directory/folder/AnkaCoder-r.ttf(Anka/Coder)` because it assumes the
directory goes up to the last slash and takes `Coder)` as filename.

But instead of using the concrete subfont name we can also specify the
index of the subfont in the font file, and that circumvents the problem.

[note]
The TTC opening feature is not well documented in Fontforge, best
described here:
https://fontforge.org/docs/ui/menus/filemenu.html#the-file-menu

Fixes: #1879

Reported-by: @oktoling
Signed-off-by: Fini Jastrow <ulf.fini.jastrow@desy.de>

font-patcher

@@ -6,7 +6,7 @@
 from __future__ import absolute_import, print_function, unicode_literals
 
 # Change the script version when you edit this script:
-script_version = "4.20.3"
+script_version = "4.20.4"
 
 version = "3.4.0"
 projectName = "Nerd Fonts"
@@ -2273,7 +2273,7 @@ def main():
           print("\n")
           logger.info("Processing %s (%d/%d)", subfont, i + 1, len(all_fonts))
         try:
-            sourceFonts.append(fontforge.open("{}({})".format(args.font, subfont), 1)) # 1 = ("fstypepermitted",))
+            sourceFonts.append(fontforge.open("{}({})".format(args.font, i), 1)) # 1 = ("fstypepermitted",))
         except Exception:
             logger.critical("Can not open font '%s', try to open with fontforge interactively to get more information",
                 subfont)