Add cgltf_accessor_data() helper

[kevinmoran]

When fetching data from an accessor there is this common code:

const uint8_t* element = cgltf_buffer_view_data(accessor->buffer_view);
if (element == NULL)
{
	return 0;
}
element += accessor->offset;

I propose adding a helper function cgltf_accessor_data() to reduce this boilerplate - I actually had a hidden bug in my code because I was not checking if buffer view data was null before adding accessor offset, so I think it's worthwhile even if it adds an extra null check.

I ran python test_all.py with no errors.

[kevinmoran]

Technically a similar helper could be added for fetching data from sparse accessors (e.g. in cgltf_find_sparse_index()), but because there's 2 buffers (indices and values) it would be uglier - either 2 separate functions or passing the return pointers as arguments. I haven't had to use sparse accessors so I don't think I'm qualified to weigh in on what would be better.