vf_vapoursynth: upgrade to VapourSynth API v4

VapourSynth introduced their version 4 API in R55, 3 years ago. mpv is
still using the deprecated version 3. I think it is good to migrate
before VapourSynth completely removes it.

The most impacted area is the video format. Previously we have a fixed
table of supported formats, each represented by an integer. In v4, the
integer is replaced by pointer to the full struct of the format.

Second, the way to create video filter is changed. Previously caller
needs to supply a "initialization" callback to `createFilter()`, which
sets up video info etc. In v4, video info is prepared first, passed to
the `createVideoFilter2()` and we get back the node.

Also, previously mpv was using the `fmSerial` filter mode, which means
VapourSynth 1) can only request one frame from mpv at a time, and 2) the
order of frames requested must be sequential. I propose to change it to
the parallel request mode, which requests multiple frames at a time in
semi random order. The reasons are, for 1), the get frame function,
`infiltGetFrame()`, unlocks the mutex during output image generation,
thus can benefit from parallel requests. For 2) thanks to the frame
buffer, unordered frame requests are acceptable. Just make sure the
buffer is large enough.

Third, the way VapourSynth scripting environment works change. In v4,
the scripting API is operated through struct pointer, much like the
exist `vsapi` pointer. The "finalize" function is also no longer needed.

DOCS/interface-changes/vapoursynth-api-v4.txt.txt

@@ -0,0 +1,3 @@
+Bump dependency of VapourSynth to utilize its API version 4. New minimum VapourSynth version for runtime is R56.
+
+Some functions and plugins are changed or removed. For details, refer to VapourSynth documentation http://www.vapoursynth.com/2021/09/r55-audio-support-and-improved-performance/ and https://github.com/vapoursynth/vapoursynth/blob/R68/APIV4%20changes.txt

meson.build

@@ -764,8 +764,8 @@ if not features['lavu-uuid']
     sources += files('misc/uuid.c')
 endif
 
-vapoursynth = dependency('vapoursynth', version: '>= 26', required: get_option('vapoursynth'))
-vapoursynth_script = dependency('vapoursynth-script', version: '>= 26',
+vapoursynth = dependency('vapoursynth', version: '>= 56', required: get_option('vapoursynth'))
+vapoursynth_script = dependency('vapoursynth-script', version: '>= 56',
                                 required: get_option('vapoursynth'))
 features += {'vapoursynth': vapoursynth.found() and vapoursynth_script.found()}
 if features['vapoursynth']