_stbt.imgutils.pixel_bounding_box doctest: Add blank row
_stbt.imgutils.pixel_bounding_box doctest: Add blank row

Just to be 100% clear that the bounding box is the biggest one, the
pixels don't have to be contiguous.

stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester