font-patcher: Allow to specify custom symbolfont with absolute path

[Finii]

[why] When one wants to use a custom symbol font and specifies it with an absolute path, the glyphdir is still prepended.

That means that the argument to --custom is always used as relative path (to --glyphdir), even when it starts with /. That is somehow unexpected or at least inconvenient.

Example: fontforge font-patcher --custom ~/Downloads/fa6.otf Inconsolata-Regular.otf

fa6.otf is searched for in ./src/glyphs/home/username/Downloads

[how] Use Python function that handles joining path fragments.

If a component is an absolute path, all previous components are thrown away.

Requirements / Checklist

• [x] Read the Contributing Guidelines
• [x] Verified the license of any newly added font, glyph, or glyph set

What does this Pull Request (PR) do?

Allow (respect) absolute paths specified with --custom. No change for relative paths.

How should this be manually tested?

Any background context you can provide?

What are the relevant tickets (if any)?

Screenshots (if appropriate or helpful)