Hello,

this is revised patch, to sum up the changes:

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.


Changes from first patch:
-Separated the Windows-specific function for getting the DLL location
from the platform-specific includes
-It is not enabled by default in configure
-The header files are not included anymore


I would like to include the header files for this reason:
While most Linux distributions ship ffmpeg, only very few contain
VapourSynth. Therefore ffmpeg won't be compiled with VapourSynth support
by these distributions, because no VapourSynth headers. Including the
headers in ffmpeg would mean they can compile with VapourSynth support
and if a user decided to install VapourSynth from somewhere else or
compile it by himself, ffmpeg support would be ready and no need for the
user to install another ffmpeg or compile one.
I'm not sure what the rules for shipping include files are, as there are
a few 3rd-party include files in ffmpeg. License is not an issue
(Vapourynth is LGPL v2.1 or later like ffmpeg).



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