observed behavior

When performing a backdrop filter blur the coverage hint is ignored when grabbing the input's snapshot. We then only blur a small subset of that snapshot in the blur passes.

Draw area of first blur subpass

For image filters this isn't a problem because the snapshot will be cropped by the hint, this is only for backdrop image filters.

expected behavior

I'd expect the blur passes to only be operating on a texture that is the size required to render the halo correctly. This will make the downsample pass use less compute and less memory. I don't expect it to affect compute used in the blur passes since they are already limited to the area where blur is happening.

cc @jonahwilliams