Update to latest STBI

We could consider moving to consuming stb as a submodule,
but for now I just added the stbi files from 067a1d1be15c5a43458d5196c91a65f48826ccb1
Makefile: Add clean target
Makefile: Use fastdep for dependencies
Merge pull request #4 from rsaxvc/master

Update to latest STBI