isnan fix
    
    isnan is already defined on Linux in math.h, as included in:
    
        toonz/sources/include/tutil.h:7,
        toonz/sources/include/traster.h:6,
        toonz/sources/include/tcolorutils.h:9,
        toonz/sources/common/tapptools/tcolorutils.cpp:4:
    
    with type:
    
        constexpr bool std::isnan(double)