Add cgltf_find_accessor helper

When using cgltf something that comes up often is the need to find the
accessor corresponding to a given attribute for a primitive (e.g.
positions or second UV channel). The code for this is simple, but it is
needed so frequently that it would be nice to include a helper for this
in cgltf proper.

cgltf.h

@@ -844,6 +844,8 @@ void cgltf_node_transform_world(const cgltf_node* node, cgltf_float* out_matrix)
 
 const uint8_t* cgltf_buffer_view_data(const cgltf_buffer_view* view);
 
+const cgltf_accessor* cgltf_find_accessor(const cgltf_primitive* prim, cgltf_attribute_type type, cgltf_int index);
+
 cgltf_bool cgltf_accessor_read_float(const cgltf_accessor* accessor, cgltf_size index, cgltf_float* out, cgltf_size element_size);
 cgltf_bool cgltf_accessor_read_uint(const cgltf_accessor* accessor, cgltf_size index, cgltf_uint* out, cgltf_size element_size);
 cgltf_size cgltf_accessor_read_index(const cgltf_accessor* accessor, cgltf_size index);
@@ -2312,6 +2314,18 @@ const uint8_t* cgltf_buffer_view_data(const cgltf_buffer_view* view)
 	return result;
 }
 
+const cgltf_accessor* cgltf_find_accessor(const cgltf_primitive* prim, cgltf_attribute_type type, cgltf_int index)
+{
+	for (cgltf_size i = 0; i < prim->attributes_count; ++i)
+	{
+		const cgltf_attribute* attr = &prim->attributes[i];
+		if (attr->type == type && attr->index == index)
+			return attr->data;
+	}
+
+	return NULL;
+}
+
 cgltf_bool cgltf_accessor_read_float(const cgltf_accessor* accessor, cgltf_size index, cgltf_float* out, cgltf_size element_size)
 {
 	if (accessor->is_sparse)