The existing unit test had an incorrect expectation in what happens
after splicing an existing range. Fixing the unit test revealed an
actual bug in the RangeMap implementation. It should never modify the
lower bound of an existing interval, because that invalidates the
iterator. It should also never retain a pointer or reference to a range
when the range is potentially removed by subsequent codelines.
This will allow the Vulkan backend to efficiently track the subresource
image layouts for each texture.
This is a sparse container for a series of ordered non-overlapping
integer intervals, where each interval maps to a concrete value.
