UI backdrop blur

## What problem does this solve or what need does it fill?

Currently, blur effects are contained within specialized effects like bloom, depth of field, UI box shadow, motion blur, etc.. A specialized blur effect would be useful for blurring the full screen behind a pause menu, or frosted glass effects for UI elements.

Users who wish to do this today would need to create custom solutions that may involve modifying existing post-processing modules or hooking into rendering internals.

## What solution would you like?

A backdrop blur module to be used by UI nodes in `bevy_ui_render` to blur the rendered content behind it, as well as facility to composite UI elements on top of a blurred scene (like GTA V's pause menu)

Ideally would use Dual Kawase blur since it's cheaper than Gaussian blur with similar results. ([great blur article here](https://blog.frost.kiwi/dual-kawase/)).

## What alternative(s) have you considered?

A more general blur which can be applied to non-UI elements. I think it's best to scope this to UI only to keep things simple for now.

Start by reading the UI rendering code in `bevy_ui_render` and the linked Dual Kawase blur article. Define how a backdrop blur for UI nodes and compositing UI over the blurred scene should fit the existing rendering flow; the work is done when those pause-menu and frosted-glass use cases are supported within the UI scope.

Written by the indexing model from the issue text.