Can apply CanvasItemMaterial to 3D Nodes

[27thLiz]

Operating system or device - Godot version:
All - Godot master (a69e449)

Issue description:
In the inspector, for any GeometryInstance derived node, you can falsely set a CanvasItemMaterial as the material_override property. Doing so will crash the game at runtime, the editor keeps working.

Steps to reproduce:

• Add a TestCube to a scene.
• In the inspector, click on the Material Override property. You can choose to create a new CanvasItemMaterial

[27thLiz]

Closing, as CanvasItemMaterial has been removed by recent changes to how shaders work.
Now every Node type uses ShaderMaterial and the type is specified in the shader code.