add load bars for <img> els in <a-asset-items>

[kylebakerio]

https://stackoverflow.com/questions/14218607/javascript-loading-progress-of-an-image

[kylebakerio]

while I generally recommend using a-asset-item for everything, this seemed to produce an error when using it with a-curvedimage, though I suspect that's fundamentally a bug in a-frame or THREE

[kylebakerio]

looks like getting load bars for images is complex from looking into it; errors should be handled now, but we could still make this more elegant in the load screen.