Deoldify Vapoursynth filter

[Selur]

@Dan64: Adjusted Hybrid to set sc_thresholds minimum to 0 when DeepEx is enabled and 0 otherwise. Through this, sc_threshold will be at least 0.01 when DeepEx is enabled.
Not checking for images when '... external ...' is used, since to do it seriously one would have to check that all images:

• exist
  
• are really pngs
  
• their image resolution is correct (= matches the resolution at the place DeOldify is used)
  
• their naming scene is correct and the frame count fits into the current clip
  

=> Not doing that, since I think folks that use '... external ...' should know what they are doing.
(updated the deolidfy test download)

Cu Selur