feat_import uses test framework set_materials to configure texture resulting in large numbers of redundant texture nodes

[ian-m-carr]

the feat_import branch uses the unit test code to create blender objects.

Currently create_material takes the texture input but does nothing with it, instead the texture is handled in set_material.

set_material creates the texture node and links it into the material on every call, this results in a redundant node in the material for every prior call.

Moving the texture initialization to the create_material method (where it is documented) and only runnning it if the material does not exist removes the redundant re-initialization of the texture node.

Note: this change may effect the unit tests, separating the code for the import and unit tests would mitigate this risk.

Change in my fork:
5ba487c