Optimize rendering of a geppetto instance when it's already in the geppetto model

When adding an already existing geppetto instance (containing a obj file) , e.g. the instance is already on a canvas, then we could benefit from reusing the already computated obj file to the mesh (from the previous canvas)

see also #163

Start by tracing how a Geppetto instance containing an OBJ file is rendered onto a canvas, then compare the behavior described in issue #163. The change is complete when an instance already present in the Geppetto model can reuse its previously computed OBJ-to-mesh result instead of recomputing it for another canvas.

Written by the indexing model from the issue text.