- Update to 51:
* updated visual studio 2019 runtime version
* fixed a cache shrinking issue
* fixed a crash when removing a message handler without a free function
* fixed an extremely rare threading issue only affecting
fmparallelrequests filters and arframeready events
* fixed compilation when avs+ master is used
* fixed lut and lut2 triggering a fatal error when invalid
planes were specified
* fixed property append operations on non-empty keys not properly
copying the underlying data
* fixed wave64 headers generated by avfs
* fixed crash in averageframes with odd number of clips
* scale averageframes for integer chroma by distance from grey
* several fixes and improvements regarding handling of the active
script environment in python
* plugin loading now has better error messages
* using get_core() in python now generates a deprecation warning
since it's been deprecated for years