Consistent with Skia, we should add a DlImageFilter to the saveLayer operation to enable backdrop filtering.

saveLayer seems like an odd place for it, but it works reasonably well because a backdrop filter will necessarily involve the same context switching that happens in a saveLayer. It is essentially initializing a saveLayer temporary image with a filtered copy of the destination (optionally allowing children to paint into that buffer) and then blitting it back to the original surface.