IECoreHoudini : FromHoudiniGeometryConverter - faster UV welding
IECoreHoudini : FromHoudiniGeometryConverter - faster UV welding

- we had a O(N ^ 2) performance on writing indexed UVs.
- Use an unordered_map to accelerate UV to index lookups instead.

appleseedhq/cortex,appleseedhq/cortex,appleseedhq/cortex