[reftests] Use requestAnimationFrame instead of takeScreenshotDelayed

[kainino0x]

takeScreenshotDelayed(50) is a bad heuristic for making sure rendering is done before taking the screenshot:

cts/src/webgpu/web_platform/reftests/gpu_ref_test.ts

Line 24 in d863101

takeScreenshotDelayed(50);

See discussion: https://github.com/gpuweb/cts/pull/4479/files#r2446286603

Instead we should replace it with something that uses requestAnimationFrame, which won't fire until (just before) rendering is actually happening, then take the screenshot. Something like this (or equivalent using async) would be theoretically tightest:

function requestPostAnimationFrame(fn) {
  requestAnimationFrame(() => timeout(fn, 0));
}

requestPostAnimationFrame(takeScreenshot);

EDIT: But see below, we need to do rAF(rAF(fn)) to be safe.

Also, might try to remove other usages of takeScreenshotDelayed so that it's private to runRefTest.

[szager-chromium]

requestAnimationFrame(setTimeout(fn)) is theoretically tightest, but it's not reliable in Webkit due to some subtle architectural differences in the engine. rAF(rAF(fn)) is known to work well in all major implementations. For examples of usage see wpt/intersection-observer/.

[kainino0x]

Thanks for the info, good to know so we don't have to find out by trial and error!

Actually while I'm thinking about it, @mwyrzykowski @ErichDonGubler are y'all running the reftests? It should happen automatically if you're running through WPT, but Chromium runs only the reftests through WPT and everything else separately.

[mwyrzykowski]

Oh I didn't even realize these existed, thank you for alerting me @kainino0x

So for some reason we are intentionally deleting them from our local copy:
https://github.com/WebKit/WebKit/blob/8a7e32536bc5938e07a1d701e3cf59a4c1204ac3/Tools/Scripts/import-webgpu-cts#L96

I will take a look to see why that is and add them back to WebKit if there is no good reason for doing that.