Fixes crash on editing curve with control point tool
    
    OpenToonz crashes when directly manipulating curves via the lines
    themselves, unless the curve was initialized as a straight line.
    
    Depending on the build environment, a calculation interprets doubles
    as ints, resulting in a divison by zero.
    
     -Explicitly specifies the use of floating point abs() - fabs()
    
    Resolves: #858, #919