glTF export: Fix submesh indices handling

[alexchuber]

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

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