If present, only use alpha channel for bounding box

[radarhere]

Resolves #4453

Changes getbbox for modes RGBA, RGBa, PA, LA and La.

Testing, I find that the current functionality is to trim black transparent pixels. This PR changes it to trim any transparent pixels, regardless of color.

[hugovk]

Let's include this change in the release notes.

[hugovk]

Thanks for the release notes. Shall we use headings like "API Additions", "API Changes", etc?

• https://pillow.readthedocs.io/en/stable/releasenotes/index.html

[hugovk]

Thanks!