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

[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.

Signed-off-by: Fini Jastrow <[email protected]>

font-patcher

@@ -225,7 +225,7 @@ class font_patcher:
                     if symfont:
                         symfont.close()
                         symfont = None
-                    symfont = fontforge.open(self.args.glyphdir + patch['Filename'])
+                    symfont = fontforge.open(os.path.join(self.args.glyphdir, patch['Filename']))
 
                     # Match the symbol font size to the source font size
                     symfont.em = self.sourceFont.em