Detect vertex count increases after Draco compression for both uint8 and uint16 indices

[donmccurdy]

Discussed in KhronosGroup/glTF-Blender-IO#2162 (comment), I think the current logic for detecting vertex count increases after Draco compression (which not predictable, we have to check after compression) currently handles uint16 but not uint8, I'd like to check whether that's intended and probably add a case for uint8 indices as well.