CMake: Use improved optimization flags on MSVC

While not having a noticeable effect on CPU-bound applications, this
change gives an about 30-50% increase in performance for games using
the GPU.
CMake: Silence PNG not found error

Hopefully this will make people stop thinking it's a hard dependency.
Merge pull request #325 from yuriks/cmake-opts

CMake: Turn MSVC optimizations up to 11