Set force_opaque when adding annotations to Brain

[wmvanvliet]

Similarly to #286, there is a render bug when adding atlas annotations on some chipsets. The solution is the same: set the force_opaque=True flag on the surface actor. Despite its name, this doesn't seem to interfere with setting the alpha for the annotation overlay.

Before this PR:

After this PR:

Playing with transparency: brain.add.annotation('aparc', alpha=0.3)