Right now GPUExternalTextures are guaranteed to be valid until the end of the next RAF. This might cause issues for example for backgrounded tabs that don't run RAFs. I kind of remember this issue being brought up before but I don't see any open issue for it, so here it is.

I suggest that we change this to say that GPUExternalTexture may only change expired at microtask boundary. Such that if you check a GPUExternalTexture expiredness and submit it in the same microtask, things are guaranteed to work.

CC @shaoboyan @kainino0x