* update to run-vcpkg@v6
* bump to cmake 3.18
* use newer cuda syntax, fail if cuda enabled but not found
* update win build script
* improve travis with non-cuda-enabled builds
* improve windows building process
collateral fixes:
* handle opencv4 upgrade on vcpkg
* add yasm (ffmpeg prerequisite) on travis
* install ffmpeg separately from opencv to circumvent time limits
* build only release to halve ci time
* do not build as shared libs when using vcpkg, since default triplets are static
* remove debug configs from appveyor matrix
macOS
[build.sh] improve handling of vcpkg
[readme] update instructions for newer visual studio versions
[build.ps1] add VS2015 and VS2019 using vswhere