fix doctest in base.pyx #7985
Description
The following doctest from line 757 of base.pyx:
sage: G = tetrahedron(color='red') + tetrahedron(color='yellow') + tetrahedron(color='red', opacity=0.5)
sage: G.texture_set()
set([Texture(texture..., red, ff0000), Texture(texture..., yellow, ffff00), Texture(texture..., red, ff0000)])
will fail sometimes because the order of the elements of the set is not fixed. The attached patch fixes this by outputting first the two red textures, and then the yellow texture.
Component: graphics
Author: Bill Cauchois
Reviewer: Willem Jan Palenstijn
Merged: sage-4.3.1.rc1
Issue created by migration from https://trac.sagemath.org/ticket/7985