Blob images allocate more pixels than needed if there's a dirty rect #3320
Description
While finding my way in between the various coordinate spaces of the blob image recoordination I noticed that blob images allocate bytes for the full image or tile and then only paint the pixels for the dirty region.
Doing this makes the code a tiny bit simpler for the resource cache since we end up with an update that is similar to how we deal with regular image updates but it means we spend memory and CPU on pixels that won't be read.