Yield CPU between drawing a frame and decoding the next one
premultiply constants in yuv conversion

probably doesn't make a difference, but it can't hurt eh :D
Copy decoded frame into output buffer faster

Was previously attempting to use imagData.data.set() incorrectly;
have to set it from another array, not directly from an ArrayBuffer.
This speeds up output a little on Safari, Chrome, and Firefox.

IE still can't use this technique due to using the old-fashioned
CanvasPixelArray instead of Uint8ClampedArray.