CMake: Turn MSVC optimizations up to 11

This improves performance on GPU bound applications by about 30% to 50% depending on the configuration:

 | master | cmake-opts
--- | --- | ---
Release | 7.4212 s | 5.6802 s
RelWithDebInfo | 8.9296 s | 5.731 s
There should still be some sort of feedback if PNG can be found. some message like "yo, couldn't find png, but don't worry"