Fix bug on Mac OS X PPC platform

For some reason the OpenGL line type (stipple etc) was causing point
sprites not to render. Resetting back to a solid line fixes the issue
and is reasonable to do (restore expected state). Thanks to Sean
McBride for providing access to the failing machine.

Change-Id: Ia31ae087e4d830175a194643c4056fd4cc68208a
Merge topic 'plot-colors' into master

ab5a0856 Fix bug on Mac OS X PPC platform