Some fixes
Squashed 'third_party/stb/' content from commit aaea13b

git-subtree-dir: third_party/stb
git-subtree-split: aaea13b71c7c810893f4c8d2396ceb48413eaed3
Merge commit '9a7293b736249d1f1dddcb1dddf8b5ce3d31d086' as 'third_party/stb'
Demo is saving to PNG instead of BMP (with stb library)