Skip Skrifa lookup for cached outlines

[valadaptive]

Stacked on #542; the last commit is the new one.

Previously, drawing a glyph performed the following lookups, in order:

1. A skrifa::BitmapStrikes lookup
2. If that fails (which it probably will, because most fonts are not bitmaps), a skrifa::ColorGlyphCollection lookup
3. If that fails (which it probably will, because most fonts are not COLR), a skrifa::OutlineGlyphCollection lookup
4. An outline cache lookup

The Skrifa lookups are somewhat slow. I believe they perform a binary search on the raw font data internally.

We can optimize this by:

• Checking for an outline glyph first, since most glyphs are outline glyphs
• Checking the outline cache without ever going through Skrifa

To do this, I refactored the prepare_[kind]_glyph functions to return an Option; this allows them to perform the requisite Skrifa "does the glyph exist" lookups themselves. The prepare_outline_glyph function can then check the outline cache itself, before doing a potentially-expensive Skrifa lookup. I also changed the outline cache to store Option<OutlinePath>, so we can store absent glyphs in the cache too.

Combined with the changes from #542, this is a ~10% speedup above current main, at least on my machine.