Don't copy video chunk data

[jprochazk]

#7481 (comment)

Even after #7860 we still copy video data for each sample. But Blob is a Arc<Vec<u8>> internally, so we should be able to avoid that as well. The decoder could receive a sub-slice of the Blob instead of copying the data to its own Vec. For web, it's possible to give the JS video APIs a direct reference to Wasm's linear memory, using the unsafe Uint8Array::view, which means constructing the EncodedVideoChunk will not require a copy either.