[webgl]Allow constructor to take canvas.

[lina128]

This feature allows users to use their own canvas, so that they have full control for downstream image processing. This feature can be used both in main thread and workers. When used in workers, user can pass an offscreenCanvas to worker. Note that, we recommend doing all the processing work (ML and image processing) in one context and one worker. Users should only render to canvas in the last step, which is lossy (canvas can only render uint8 data ranging from 0 - 255). We support keeping data on GPU (PR: #5953), which means all the ML and image processing can happen on GPU without having to download data to CPU and re-upload.

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This change is 

[pyu10055]

Reviewable status: complete! 1 of 1 approvals obtained (waiting on @lina128)

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tfjs-backend-webgl/src/backend_webgl_test.ts, line 759 at r1 (raw file):

  beforeAll(() => {
    flag = tf.env().getBool('WEBGL_CPU_FORWARD');

webgl_cpu_forward flag is not set to false?

[lina128]

Reviewable status: complete! 1 of 1 approvals obtained (waiting on @pyu10055)

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tfjs-backend-webgl/src/backend_webgl_test.ts, line 759 at r1 (raw file):

Previously, pyu10055 (Ping Yu) wrote…

webgl_cpu_forward flag is not set to false?

In nightly, we test cpu_forward = true too.