glTF export: Fix submesh indices handling

[alexchuber]

From https://forum.babylonjs.com/t/multi-materials-cause-strange-glb-export-artifacts/60558.
Fixes bug with submeshes where indexStart was improperly handled when creating the buffer view.
While in the area, I did some contained refactoring. The main points are:

• No more offset parameter. I think this was a remnant from before the rework last year, as I couldn't tell what it would've been used for.
• As little copying as possible, which also means subsetting ahead of time.
• Miscellaneous clean up

[bjsplat]

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[bjsplat]

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[sebavan]

LGTM but prefer to wait on @bghgary review :-)

[clevengermatt]

Thanks for the quick work on this issue! Any idea when this might make it into a release? My team is locked into our current version until we can get one with this issue resolved.