Cache loaded fonts

[tybulewicz]

ImageFont.load_default(size=...) is invoked per BoundingBox instance, which can become expensive for many detections on large images. Consider caching fonts by font_size (e.g., a small module-level or class-level dict / functools.lru_cache) and reusing the same ImageFont object across instances with the same size.

Originally posted by @Copilot in #482 (comment)