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
- using std::string;
- using std::wstring;
- using std::ostream;
- using std::istream;
- using std::iostream;
- using std::ostrstream;
- using std::istrstream;
- using std::fstream;
