Fix lighting overdrive on ImagePlaneWidget

ImagePlaneWidget was setting Ambient to 1.0 but leaving diffuse
at 1.0 as well. This exposed a difference in the backends where
the old backend truncated to 1.0 then modulated by texture and
the new modulates by texture and then truncates to 1.0. Fixed by
explicitly setting diffuse to 0.0 which was the intended effect.

Change-Id: I3ac1e6862e4fce6ff7228fee6b586307cc096d80
Merge topic 'spline_widget' into master

029f8bfa Fix lighting overdrive on ImagePlaneWidget