Merge branch 'Add-GradientBackground' into 'master'

src/Cxx.md

src/Cxx/Rendering/CMakeLists.txt

src/Cxx/Rendering/GradientBackground.cxx

src/Cxx/Rendering/GradientBackground.md

src/Python.md

src/Python/Rendering/GradientBackground.md

src/Python/Rendering/GradientBackground.py

...Testing/Baseline/Cxx/Rendering/TestGradientBackground.png

...ting/Baseline/Python/Rendering/TestGradientBackground.png

src/Testing/Data/star-wars-vader-tie-fighter.obj