improve multi-family output in font desc resolver

[chearon]

This bug can be understood without considering the changes from #1987. When the user adds multiple TTFs and assigns them the same family name, we send the system families from the TTFs in a comma separated list, but we weren't correctly ordering that system-family list by the best matches on the user-description side. #1987 exposed it because it almost guarantees each file will have a unique system family.

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If the user registers fonts like this:

registerFont("OpenSans-Bold.ttf", {family: "Open Sans", weight: 300});
registerFont("Arimo-Regular.ttf", {family: "Open Sans", weight: 600});

The font_face_list will look like this (approximated system descriptions):

OpenSans-Bold.ttf	sys desc	user desc
family	Open Sans	Open Sans
weight	600	300

Arimo-Regular.ttf	sys desc	user desc
family	Arimo	Open Sans
weight	400	600

When the user does this:

ctx.font = "600 Open Sans";

We should send this to the OS:

Arimo-Regular.ttf	sys desc
family	Arimo, Open Sans
weight	400

But we were sending this:

Arimo-Regular.ttf	user desc
family	Open Sans, Arimo
weight	400

fixes #2041