Blender Git Loki

bli: generational_arena: fix: begin() points to `EntryNoExist`

When the first element of `Arena::data` is `EntryNoExist`, must iterate over the vector until the first `EntryExist` is found.

For `Arena::end()` this shouldn't be necessary because it should point to the last element + 1 anyway, so the `operator*` on this is meaningless.