Remove usage of the Theora codec

[padenot]

The WebGL conformance test suite reference a few Theora file, and after years of service, this codec is being retired from the Web.

Some context, for the two implementations that have historically supported Theora, Chromium (except on Android) and Gecko (all platforms):

https://chromestatus.com/feature/5158654475239424 (cc @dalecurtis)
https://bugzilla.mozilla.org/show_bug.cgi?id=1860492 (I speak for Mozilla here, I'm handling this)

If need be, I can provide a media file that has exactly the same characteristics and switch it here with a different codec. However, my understanding is that what we care about here are primaries / transfer function / color space / color range and then pixel format / plane arrangement, and this seems to be covered by other codecs, so we should not lose any coverage here, as far as WebGPU is concerned.

See gpuweb/cts#3358 for the WebGPU version of this.