support uni-color colormap in the render config widget

[kswang1029]

This is considered as a preparation work for #1204

The idea is to add a button linking to a color swatch like we do for the AST overlay color selector

Then users would have the freedom to form a unicolor colormap from black to a selected color (eg, cyan).

[veggiesaurus]

are we going to adjust the shader or simply append the colormap texture? 🤔

I'd quite like to adjust the shader, as this will (in future) give us an opening to allow users to create custom colormaps

[kswang1029]

are we going to adjust the shader or simply append the colormap texture? 🤔

I'd quite like to adjust the shader, as this will (in future) give us an opening to allow users to create custom colormaps

Open to any ideas that have potentials 🙂