Blur shader

[oscartbeaumont]

This PR implements a wgpu blur shader. It does not handle the integration with the editor just implementing the filter in Rust.

You must uncomment the BlurLayer::render call to preview it.

An example of it bluring part of a frame (notice the PR title):

[Brendonovich]

• blur_radius isn't being used
• Can it be done in one render pass?
• random noise to avoid un-blurring?

[EChambeee]

What is the purpose of blurring part of the frame, and how can you do it?