Restore ability to scale gl2ps linewidth/pointsize.

The functions:

static void vtkGL2PSExporter::SetGlobalLineWidthFactor(float);
static void vtkGL2PSExporter::SetGlobalPointSizeFactor(float);

were removed during the GL2PS rewrite. This patch restores the
functionality by adding LineWidthFactor and PointSizeFactor ivars to
vtkGL2PSExporter and the associated setters:

void vtkGL2PSExporter::SetLineWidthFactor(float)
void vtkGL2PSExporter::SetPointSizeFactor(float)

These attributes are specific to the vtkGL2PSExporter instances, while
the old implementation stored them in global variables.

A vtkOpenGLGL2PSHelper is added to RenderingOpenGL that fakes the
GL2PS function calls needed to update line width, point size, and
stipple state. The public static methods are called from the rendering
code when the GL state is changed to inform GL2PS of the change. The
vtkOpenGLGL2PSHelper class is used to update GL2PS without introducing
a dependency on vtkgl2ps to vtkRenderingOpenGL.

The TestContextGL2PS unit test is modified to ensure that 1.0 scaling
factors are used.

The vtkGL2PSContextDevice::StippleOn was not used, and is removed in
this commit.

Change-Id: Ia367211848f69ffdb093fceeb9baa029435a20d0
Merge topic 'gl2ps-size-factors' into master

6643a930 Restore ability to scale gl2ps linewidth/pointsize.