[webgl] disable offscreen canvas for safari,

[pyu10055]

gl fencing is not working yet for Safari technology preview, to reproduce

canvas = new OffscreenCanvas(300,300);
gl = canvas.getContext('webgl2');
sync = gl.fenceSync(gl.SYNC_GPU_COMMANDS_COMPLETE, 0);
gl.flush();
setTimeout(() => console.log(gl.clientWaitSync(sync, 0, 0)), 1000);

The status is always gl.TIMEOUT_EXPIRED, would never turn to ALREADY_SIGNED or CONDITION_SATIFIED.

fixed #7364

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This change is 

[Linchenn]

Thanks for fixing it! cc @qjia7, as Ping found, Safari meta added support of Offscreen canvas but the fencing API does not work as mentioned in the PR description.

[mattsoulanille]

LGTM

[stevexbritton]

This is not fixed in the latest STP 165 released today.