[Impeller] Expensive IO/Worker tasks can slow down raster thread work. #135443
Description
Even after #134452 , sufficiently complex background work can slow down the raster thread work. For some reason, this is actually worse on Vulkan than on GLES. I've deduped a number of other bugs into this instance. While the immediate fix is to use appropriately sized images, I'm filling this as a long term issue to better understand our options here:
