gltf: Permit sparse accessors without a bufferView.

modules/gltf/doc_classes/GLTFAccessor.xml

@@ -7,7 +7,7 @@
 	<tutorials>
 	</tutorials>
 	<members>
-		<member name="buffer_view" type="int" setter="set_buffer_view" getter="get_buffer_view" default="0">
+		<member name="buffer_view" type="int" setter="set_buffer_view" getter="get_buffer_view" default="-1">
 		</member>
 		<member name="byte_offset" type="int" setter="set_byte_offset" getter="get_byte_offset" default="0">
 		</member>

modules/gltf/structures/gltf_accessor.h

@@ -39,7 +39,7 @@ struct GLTFAccessor : public Resource {
 	friend class GLTFDocument;
 
 private:
-	GLTFBufferViewIndex buffer_view = 0;
+	GLTFBufferViewIndex buffer_view = -1;
 	int byte_offset = 0;
 	int component_type = 0;
 	bool normalized = false;