Separate muxed/memory fonts from system fonts

[rcombs]

It'd be useful to explicitly distinguish fonts that were muxed into the video file from fonts that we retrieved from a provider. By default, I think we should always prioritize name-matching memory fonts over system-provided fonts, even if the system-provided one is a closer match metrics-wise.

This should help protect against cases where a file's author had one weight of a font installed (and muxed it), but the user had another that matched the requested metrics more closely (this is quite common with weights, as typesetters often don't set \b to match the weight of the font they're requesting; there've also been cases where an author used faux-italics, not realizing that an actual italic version of the same font also existed).

This is technically distinct from VSFilter's behavior, but font provider stuff always is; this just affects the "effective set of installed fonts".