Add a rectangle frame around 2D plots and pixel grid in debug mode

[grzanka]

For some tests we would like to use small cubes (i.e. 3x3x3 voxels). When user runs pytripgui in verbose mode (-v option from command line) some additional things should be plotted on the image:

• red(?) frame around 2D plot to depict plot border
• grid lines showing pixel borders (maybe in very verbose mode: -vv)