Modelviewer tweaks

- Print a warning (modelviewer only) when a .model does not define a material used by mesh
- Print a warning for missing UV coordinates, but fill them with zero instead of aborting load
- Last but not least, landing pad & scaling helper can be toggled with p:

<img src=http://i.imgur.com/tHGrJ0E.png />

100x100m landing pad, 1.8m tall box people and top/side silhouettes of a space shuttle (37.24m long)