Tiles Traversal: Validate handling of the "Add" refinement

[gkjohnson]

Related to #669

• The recursivelyMarkUsed and recursivelyLoadNextRenderableTiles function both don't account for "ADD" refinement when determining whether to continue traversal whereas both functions are designed to support loading all "REPLACE" siblings. It's likely a rare case that a REPLACE tile has empty children and then an "ADD" child, though.
• The "markVisibleTiles" (see here, here) function assumes that the "ADD" refinement qualifier refers to the current tile rather than the behavior for the children The current behavior is correct per Clarify Additive Refinement Behavior CesiumGS/3d-tiles#416 (comment).