Using File->Save Image menu didn't work while Stars were drawn... the used
method to save an image (mainqenter F3 key) is a bit awkward anyway, so
replaced with proper BIF_xxx call.
Reason for mainqenter not to work was RE_draw_stars() function calling
a blender_test_break(), swallowing queue events. Very bad and need review.
Small tweak in previewrender; preview type Cube now displays texture
coordinates better (it showed a bit too much)