Divide by zero error in matplotlib.tests.test_colors.test_light_source_shading_color_range

[jenshnielsen]

This is mainly to remind my self to fix this later. Not release critical

A bunch of Numpy runtime warnings are thrown in this test because it is shading
a flat surface which gives an uniform intensity which results in a divide by zero when normalising the intensity.

This should be fixed IMHO by:

• Add a test with a more interesting surface.
• Change share_rgb to avoid dividing by (intensity.max()-intensity.min()) when these are identical. This will change the result of the current test.
• Alternatively one could fix this adding an assertion and refuse to shade flat surfaces.

[jenshnielsen]

This has been fixed on the master branch by the merge of #3291