Fix compilation of shaders during build

[mchaptel]

Should probably use fxc to compile FX/vso/pso files during build process. It would allow to compile with optimisations and provide users with a smooth install experience.

A solution would be to add an optional argument to the build.bat to force recompile using fxc.exe. it would be run during ci, and could be ignored by developers who just want the game to recompile a couple of shaders.

Also should investigate if xcopy can preserve the modified date as it might be the issue.