R61

• fixed error message when splicing mismatched clips so it's actually helpful
• fixed a memory leak when freeing setframeprops filter (Endill)
• fixed planestats being broken for float format (Endill)
• fixed makefulldiff for 16 bit clips, previously it used too small intermediate variables
• blankclip can now emit variable size and variable format clips
• libraries can now register clean up callbacks when a core gets destroyed