Image resize: Add trim() for "boring" pixels

Competing image resize services will trim all “boring” pixels from the edges of an image:

This is VERY useful for situations like product images, where the transparent or white background edges need to be trimmed off automatically. It would look at the top left pixel color and trim that color off of all edges, as much as needed. Cloudflare’s current version appears to only trim a specified # of pixels from the edge.