Bring airplane down to earth

Our .dae export puts the airplane 100m in the air. Make an adjustment so it's
in the right place.

Signed-off-by: Casey Dahlin <cdahlin@redhat.com>
Introduce private transforms for objects

Private transforms apply only to the object itself, not its children.

Signed-off-by: Casey Dahlin <cdahlin@redhat.com>
Fix COLLADA handling of non-Y-up files

Blender likes Z to mean Up, OpenGL likes Y to mean up. COLLADA supports any of
the above. Our previous ham-fisted attempts to deal with it have been
corrected. Our Z-up aeroplane now draws correctly with its native
transformation.

Signed-off-by: Casey Dahlin <cdahlin@redhat.com>