fix: misc

[scarcekoi]

• Add tests
• Replace open_image macro with plain function
• Add angle/rotation option (Rotation/Angle Option #44)
• Handle transparency (shadows) in images with composite layout (Handle transparency (shadows) in images with composite layout #30)
• Update to Rust native webp implementation (Update to Rust native webp implementation #5)

[uncenter]

I don't think it makes sense to tackle all of these issues in a single "misc" pull request - better to split it up and make targeted, scoped changes for each problem you intend to fix.