Bug 1104270 - fix eager font and image parsing

That's no longer the case, no. Image bytes are only decoded when that's actually needed - in all other cases, we just use the `HTMLImageElement` we created from the blob. Iff we need the pixels because content uses, say, `getPixel`, we decode into a buffer, so are guaranteed to have uncompressed pixels here. Having the compressed pixels wouldn't be useful at all: there's not really anything `BitmapData` can usefully do with them.