shader: Force two-side color emission only if it is really needed
Changing a shader key will force virglrenderer to recreate the shader the next time it is looked up, and this is actually not needed if each emitted back color also has a corresoinding front color. So restrict forcing two-sided coloring to the cases where a back color is emitted without a front color.
Fixes #130
After today, I'll not be around until Wednesday, so feel free to push this if the fix looks good to you.