[merge] allow passing in TTFont, change to use argparse

[NightFurySL2001]

Implements #3614

[behdad]

I have a vague memory that the merger doesn't take TTFonts, because it has to load fonts multiple times. But I'm probably wrong. I don't have time to review this currently.

[NightFurySL2001]

From what I saw the fonts are loaded multiple times to build a new GlyphOrder. Tried to sidestep it by using deepcopy (although I believe it's not required at all).

[behdad]

From what I saw the fonts are loaded multiple times to build a new GlyphOrder. Tried to sidestep it by using deepcopy (although I believe it's not required at all).

I don't think that would work. Se want to reload the fonts using the new GlyphOrder.

[NightFurySL2001]

Pretty sure the reload happens before setting the new GlyphOrder. No other files call for _openFonts() in merge module. https://github.com/fonttools/fonttools/blob/770917d89e9151023cce011c7f7223f4ce84169e/Lib/fontTools/merge/init.py#L79-L91

[behdad]

The font's GlyphOrder is loaded lazily. After the first _openFonts, the GlyphOrder of the font is used, which means cmap and some other tables are loaded.

We load the font a second time, and set GlyphOrder on it before accessing any tables, so all tables are loaded using the new GlyphOrder...