[Ray-tracing] Change index offset to first index

[Vecvec]

Connections
Mentioned on matrix as being confusing.

Description
index_buffer_offset is validated that the offset is divisible by the size of the index type, this suggests that it is probably better - and less confusing - if this is changed to be called first index and is multiplied by the size of the index type.

Testing
This is a change mostly to naming - it shouldn't need any new tests.

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy. If applicable, add:
  • [n/a] --target wasm32-unknown-unknown
  • [n/a] --target wasm32-unknown-emscripten
• Run cargo xtask test to run tests.
• Add change to CHANGELOG.md. See simple instructions inside file.

[cwfitzgerald]

Looks good and matches how draw calls work