avm2: Refactor draw_triangles_internal

[SuchAFuriousDeath]

Description

This is a step towards rendering with uvt (which I'm trying to eventually implement) and this feels like a good time to submit this chunk as a separate PR before it gets too overwhelming. The behavioral changes should be minimal, the only added thing should be validation that indices come in triples (if not, an error is thrown, I adjusted the existing test adequately). I think the code is also much more readable now.

Testing

How can we test this PR?

Everything is already included in the adjusted test.

Checklist

• I, a human, have self-reviewed this PR and fully understand the changes within.
• I have made or updated tests where possible.
• All of my commits are properly scoped, compile successfully, and pass all tests.
• This PR does not make sense to split up into smaller PRs.
• An LLM was involved in the authoring of this code.

[kjarosh]

Damn, I had some triangles refactor that was improving performance and fixing winding behavior, which will conflict with it :/

It's no big deal, but probably can help in implementing UVT.