Because the `onFrame` callback could do async work, it was possible for onDone to try to submit the costumes before the onFrame had produced any. This was subject to browsers different timing treatments of setTimeout vs. promise resolution. This makes the gif-decoder use the same style of progress API as the file uploader

randomize sprite positions when added from image file or from library (through 'choose from library' and 'surprise')

And start just creating assets to hand to the VM. Towards https://github.com/LLK/scratch-vm/issues/1577

Update calls to addSprite2 to use new API for adding sprites. Cleanup sprite3 representation for uploading a new sprite from an image.

Firefox and Safari both process mp3s as having audio/mpeg, and a variety of different audio/wav* possibilities.