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.
- 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
- 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.
