Add plane and cube shapes to material viewer

[zndxcvbn]

Is your feature request related to a problem? Please describe.

This innovation will help you better see .vmat textures and other texture representations

Describe the solution you'd like

Add plane and cube shapes, and also add a button to the interface where you can select a shape to view the .vmat texture

Describe alternatives you've considered

No response

[xPaw]

I wonder how we could detect which shape would be best to show by default for a given material.

Also doesn't a cube practically cover the same use case as a cube? Perhaps we could just show both a cube and a sphere?

[xPaw]

To add to this, some models have a "PreviewModel"

For example gloves/paints/bloodhound_snakeskin_brass.vmat_c has this in DATA:

    m_stringAttributes = 
    [
        {
            m_name = "PreviewModel"
            m_value = "characters/models/shared/arms/glove_brokenfang/v_glove_brokenfang.vmdl"
        },
    ]