Add workaround for webgpu blur backend

[shaoboyan]

WebGPU blur sample exposes a GPU memory leak issue in WebGPU.
When using WebGPU canvas as source input to VideoFrame, the GPU memory leaks.

This PR add a workaround for this issue by reading gpu data back to CPU and
use 2D canvas as VideoFrame source input.

Description

Purpose

[shaoboyan]

@huningxin @Jiawei-Shao PTAL, thanks!

[huningxin]

Thanks @shaoboyan!

[shaoboyan]

@huningxin and @Jiawei-Shao PTAL again, thanks!

[huningxin]

LGTM, thanks @shaoboyan !

Just want to confirm with you whether your cl 3544653 fixes this issue in Chromium?

[shaoboyan]

Yes, I did some test locally. Hope you could also have a try when it is landed.

[huningxin]

@shaoboyan , thanks for your work on the workaround and the chromium fix.

I tested your PR, it fixes the memory leak issue. However, I found FPS drop (30FPS -> 20FPS) after applying this workaround. I guess it is because memory is readback to CPU for processing. As you know, this sample is mainly for the investigation of the full-GPU-only video processing pipeline, ideally it should keep the data on GPU along the way.

Given the chromium fix has good progress, I would propose to merge this workaround into a separate branch. People can use it if they meet the memory leak. And once the chromium fix is landed, I'll let people know and use the latest Chromium.

WDYT?

[shaoboyan]

@huningxin I think it is Ok for me if people are using it. And I think that CL should be landed soon.