Fixed model cache bug

[lilleyse]

Fixes #5720

While the discussion in #5720 is all valid, the fix was simpler. This prevents the pipeline extra's source buffer from being removed when loading completes. This allows any future models to reference that same buffer.

I had to restructure one test which was doing a clone on the .gltf property, since our clone function doesn't work for typed arrays.

[mramato]

Are you sure this completely fixes the bug? Won't this still result in some model resource being loaded twice for the reason I discussed in the linked issue?

[lilleyse]

This fixes the crash in the Sandcastle code and should fix possible other cases. Pre-2.0 we did something similar and held onto the bgltf buffer directly inside the gltf cache. Now the buffer data is just referenced by the gltf json.

Render resources do get created twice, but I'm pretty sure that's expected since the 2nd model is updated after the first is destroyed. If the 1st model is not deleted, both the gltf cache and renderer resources are created once.

[mramato]

I'll take your work for it, but I still think the model caching code overall could use a lot of cleanup.

@moneimne can you please review this? @hpinkos please merge when happy.

[moneimne]

Seems fine to me!

[hpinkos]

Yeah, this doesn't seem like the best solution, but it will work for now