Deduplicate fontdb types

[dhardy]

resvg optionally depends on fontdb. Meanwhile,

• usvg::FontStyle is identical to fontdb::Style
• usvg::FontStretch is identical to fontdb::Stretch

Worse, there isn't even a conversion method between the two. This is a pain when implementing a usvg::FontResolver.

[RazrFalcon]

Erm... what exactly is your solution?

Also, there are like 2 people who would need to implement usvg::FontResolver themselves. It's a super niche case.

[dhardy]

The simplest I can think of is to make this feature dependent:

• when usvg uses fontdb, those types are just re-exports of the fontdb types
• otherwise, they are distinct types

[RazrFalcon]

This will only complicate things.

[dhardy]

No, it will simplify things for anyone downstream happening to need to use both types.

An alternative would be to implement From conversions. Probably more robust (to unforeseen changes in fontdb definitions of the types), but slightly more complicated.

[RazrFalcon]

From is fine.