* added couple of tests to avoid crashes
* changed scene_viewer.material's shaders so grid have colors
Added Rotate X checkbox.
If checked and Preview, script rotates scene around x -90 (and rotates it back after exporting) so node->rotateY(ang); works right.
If used, no instances anymore. Preview only when in Object-mode.
Merge pull request #6 from bosoni/master

Added checks to prevent crashes and grid colors