Hello,

this is my first patch, I hope I got all the formalities correct.

The current VapourSynth implementation is rarely used, as it links the
VapourSynth library at build time, making the resulting build unable to
run when VapourSynth is not installed. Therefore barely anyone compiles
with VapourSynth activated.

I changed it, so that it loads the library at runtime when a VapourSynth
script should be opened, just like AviSynth does.
On Windows the DLL from VapourSynth is not installed in the system
directory, but the location is stored in the Registry. Therefore I added
some code to read that information from the registry.

As the V4 API is designed with dynamic loading in mind (only a single
import), I updated the implementation to V4 (changes are mostly
superficial, no structural changes). The V4 API is already several years
old, fully supported since R55 released in 2021.

I copied the two needed header files directly in a vapoursynth.h,
removing the need to install VapourSynth on the build machine
(VapourSynth is also LGPL 2.1 or later, so no license issue). I updated
the configure so that it checks for the ability to load libraries at
runtime for VapourSynth, just like AviSynth and activate it if not disabled.

make fate runs without any issue. I tested VapourSynth input scripts
with various color formats on different platforms:

Ubuntu 22.04
macOS 13 (x86_64)
macOS 13 (arm64)
Windows 10 (msys2/gcc)

It compiles on these platforms without any warning and runs without any
issues.

Best regards
Stefan