Ported WebGPU blur path to use fragment shaders

[toji]

Based on a request from kbr@ in https://issues.chromium.org/u/1/issues/445223447

This change ports the WebGPU Blur renderer to use Render passes instead of compute passes. The shaders were ported as directly as possible from the WebGL version of the shaders, not the WebGPU version that was previously in the file. This is for the purposes of providing a more direct power usage comparison between the WebGPU and WebGL versions (which I have not done).

The main benefits of using the fragment shaders here are that they make some of the texture sampling simpler, "workgroup" sizes will be handled by the implementation automatically and optimally. It also doesn't require STORAGE usage, and thus any concerns about being able to use bgra8 as a storage texture are no longer a problem. This likely removes the need for the "Zero-copy output" toggle, since you are guaranteed that it can always be zero copy. Nevertheless I've still included support for both modes in case it was providing something else I wasn't aware of.

[axinging]

I think we should keep both graphics and compute path, so that we can have a better understanding and comparision between the two?

[toji]

webatintel/videoeffect#7 handles both use shader types, and seems to be the right repo for it.