Followup for fix T38318: Add back previous fix and fix a glitch.

The previous fix 162d6c73 has been cancelled because it caused a
problem that mesh deform cages and so on mess up the preview. To avoid
the problem, viewport doesn't show mesh object if its draw type is
bounding box or wireframe.

Reviewers: brecht

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D375