WIP [COLR] Add PaintLocation

[behdad]

Doesn't work...

[behdad]

This is near impossible to make work. Definitely not with our current abstractions and APIs.

The problem is that PaintLocation cannot just set new coordinates on hb_font_t, for two reasons:

• It would break the threadsafety of hb_font_t, which is a non-starter.
• Even if we install new coordinates on hb_font_t, then hb_font_t attachments (a cairo scaled font, etc) won't notice that the hb_font_t changed underneath them.

Another approach would be to create a new hb_font_t and install the locations on it. But in general we don't have a hb_font_create_similar(). The hb_font_create_sub_font() is not sufficient, since it won't have proper font-funcs installed on it.

So to do this, we would need to add a hb_font_draw_at_location() which would be very ugly.

Yet another approach, would be to add push_location / pop_location to the hb-paint API, but that would have its own problems I believe.

[behdad]

FontTools counterpart. fonttools/fonttools#3262

[behdad]

FontTools counterpart. fonttools/fonttools#3262

The patch in this PR doesn't quite match the fonttools code. In particular AxisList has a Format field in the fonttools impl, that this patch doesn't account for. But as I said in the previous comment, it's near impossible to make it work currently anyway. So I'm abandoning it.