This patch will show the background camera images behind a solid
The change consist out of selecting the correct texture for viewport compositing
so it has an alpha channel to write to. And a alpha under wire blending mode.
During development we needed to make a decision about what render
artifacts to allow as the blending modes as we actually needed an
blending mode to blend over the background, but blend under the wires.
In order to fix these render artifacts we can introduce a shader, but
that will require copies of the depth and color buffers.