Context: flutter/engine#39111

flutter/engine#39111 Introduced a proxy object as a way to override the color attachment pixel format from a context (CustomColorAttachmentPixelFormatContext). It was required because there was no safe way to override that value otherwise. It's not ideal since it creates an extra layer of indirection which could be confusing when debugging. We need to come up with a better way to split out the pixel format. There is hope that as we work on vulkan and offscreen buffers whose pixelformat doesn't match the surface we are going to be forced to sort this out.