Fix issues in vtkOpenGLPainterDeviceAdapter::SendMaterialProperties()

This fixes a few issues in the SendMaterialProperties() method in
the vtkOpenGLPainterDeviceAdapter class.

1. The enums for the glMaterialfv() calls were wrong when the
   data type was double.
2. Some use-cases don't need to set the specular power. Now there
   is a check for null specular_power pointer and it will only be
   set if a valid pointer is passed.

Change-Id: Ifa3e9814f87ecaec30ad3d41a9ba706469be23ca
Merge topic 'fix-send-material-properties-issues' into master

69d4df39 Fix issues in vtkOpenGLPainterDeviceAdapter::SendMaterialProperties()