Update Visual Studio project to VS 2019

[EwoutH]

Currently the Visual Studio Solution File (guetzli.sln) and the project files (guetzli.vcxproj and guetzli_static.vcxproj) are configured for Visual Studio 2015. It would be nice if those could be recompiled for Visual Studio 2019, which increases compatibility and performance.

[doterax]

You can try my fork of guetzli (with GPU support), and I have already used VS19: https://github.com/doterax/guetzli-cuda-opencl