HighColorPrecisionDrawingTest: Make robust against octave's new plotting backend.

Octave's fltk default plotting backend uses OpenGL and doesn't restore
context bindings after executing, neither do we, we are both lazy.

This means that a innocent plotting command can change the OpenGL
context on our main thread and thereby make us render into the
plot window, or vice versa, with "funny" results.

Work around this in HighColorPrecisionDrawingTest.m for now by
switching to good'ol gnuplot backend if we detect this.

The same problem may happen with future Matlab releases, so need
to implement proper protection inside Screen() soonish.

-> Fixes failure on Octave 3.8.x for now.