Provide custom renderer to make binary masks for DL training purposes

[MattClarkson]

Basic idea is:

• Create new widget
• Render scene to flat colours, no shading model.
• Use case: Just Liver
• Use case: Multiple organs, different colour each?
• Use case: Liver + LUS probe
• Use case: Multiple organs, different colour each + LUS probe

Questions:

• How to give a different pose for each model?
• How to assign colours for each model?

[MattClarkson]

• Colours will be assigned via .json file typically, so the surface_model_loader does that, as does the surface_model_directory_loader, when it has a set of surface defaults.
• Different poses can be set via set_model_to_worlds for each model, or for legacy compatibility, set_all_model_to_world will now set all models to the same model_to_world.