Still crashes when loading a video.

- Build the latest AviSynth headers using CMake, as proposed in #134
- Add and initialize AVS_Linkage to fix video loading

Video and audio playback now works in most cases, but still crashes for
some files.

This makes meson detect changes to files like default_config.json
and rebuild libresrc.cpp when necessary.

- This refers to AviSynth < 2.5.6, which is from before 2005
- With the current setup (using GetAVSLinkage, these versions wouldn't
  work anyway)
- The latest AviSynthPlus is bundled with the installer anyway

Co-authored-by: wangqr <[email protected]>

Because... let's maybe not let users make Aegisub call arbitrary
symbols in avisynth.dll/so just by editing the config.json.

Starting with AVISYNTH_INTERFACE_VERSION=5, this is how script
environments should be deleted. The previous code was causing crashes
when unloading AviSynth in certain scenarios, such as when failing to
open a file due to an incorrect path.

Since FileNotFound exceptions don't abort the provider search, opening
an invalid path would show errors such as "avisynth not found" when
Avisynth wasn't installed, even if Avisynth wasn't selected as the video
provider.

When Avisynth is not installed or not functional, this would otherwise
cause a crash when trying to initialize Avisynth more than once, since
after the first time the refcount would have been incrased anyway.

These are broken in some edge cases, such as smb mounts on Windows.
Paired with d96fc1f.

This reverts commit f5a730f.

Proper fix to the issue that f5a730f
was trying to fix.

Fixes #61 .