Support rendering with a floor

[rib]

Building on #1 when using glimpse-cli.py to render synthetic test sequences it would also be good if we could choose to render a floor if we want data that's trying to mimic what we capture from a camera when tracking in real-time.

E.g. maybe add a --include-floor boolean argument to control this.

Note: the floor shouldn't get mistaken for the body within the label images so we need to make sure that the shading for the floor will give it an unlit colour that doesn't clash with any body labels (perhaps the same color as the background, or make it invisible while rendering the color buffer somehow - only affecting the depth buffer)