Use full cache search (as in `rect_for`) to check if matching glyph
already exists. This prevents adding duplicate matching glyph textures.
Extract search logic into `GlyphSearchable` allowing more code re-use.
Split `cache_queued` into 2 phases the first splitting the glyphs into
already-cached & not-cached. The first phase does not require mutability
so can easily make use of more CPU cores.