[OpenGL viewport] Shadeless override per material, or matcap.

It would be nice in the material properties, under the "Viewport Display" options, to have a way to set this material only to be viewport shadeless .

(to not be shown in viewport with matcap or be "Flat" like it is called in the popover menu)

You already can change per material color, roughness and the metallic.

As shown in the screenshot, the use case for this is if you have a reference texture or model that you want to see in shadeless mode, but not the entire scene should be shadeless.

I could use EEVEE material preview, but it isn't the same feeling to do modeling in this mode (not correct matcap light, some noise from render when you move, and you have to modify each shader to be emissive in order to be shadeless, you need to modify it back when you are not in modeling phase)

In the title of the proposal I have mentioned matcap, it can be an option to override matcap per material (but if not easily feasible, or wrong design, the shadeless is enough for the usage)

(btw : yes my photogra is really noisy, but it was only for an angle measure and i shoot almost at night ^^)(btw2 : when vulkan integration will be finish, should we call the open gl render viewport, the Vulkan viewport render ? )