Optimize mesh indexing

[mistafunk]

Currently, the PRT encoder uses a single index for all vertex attributes (e.g. vertex normals):

palladio/src/codec/encoder/HoudiniEncoder.cpp

Line 62 in caf6676

.indexSharing(prtx::EncodePreparator::PreparationFlags::INDICES_SAME_FOR_ALL_VERTEX_ATTRIBUTES);

This leads to duplicate points like in this case https://community.esri.com/t5/arcgis-cityengine-questions/palladio/m-p/1238219

Try to optimize this by using separate indices for vertex normals.

[mistafunk]

Merged to develop.