Replace polygons with triangle strips in vtkImageSliceMapper.

The 3D image mapper displays the image as a texture on a polygon.
Rendering the polygon with GL_POLYGON is deprecated in OpenGL 3
and seemed to cause rendering issues with some drivers.  Switch to
GL_TRIANGLE_STRIP to see if it fixes these issues.

Change-Id: I5d2e3f23d4ddd26cb359f23871c1c6afe41e8c69
Merge topic 'image-triangle-strip' into master

6f063d9b Replace polygons with triangle strips in vtkImageSliceMapper.