TilesRenderer: Multi parallel loads of the same tile can result in memory leaks

[gkjohnson]

Specifically this point here:

3DTilesRendererJS/src/three/TilesRenderer.js

Lines 699 to 704 in 8142a2d

	// exit early if a new request has already started
	if ( cached._loadIndex !== loadIndex ) {
	return;
	}

If a separate load for the same tile is started, causing the first to be cancelled then we need to dispose of any "ImageBitmaps" that are left dangling. We should make sure we dispose of the content in this case.

Related to #422, #877.