Cleanup imports in examples; also fixes a micview compile error on OS X.

Note that gl.h and glu.h are always included in glut.h, and thus do not need to be included again.

Signed-off-by: Robert Xiao <brx@cs.cmu.edu>
Merge pull request #399 from nneonneo/glut-include-fix

Fix a micview compile error on OS X