make better assumptions about new color positions in a gradient

[KrabCode]

when setting 3 colors as defaults with

gui.gradient("path",
        gui.colorPoint(color(255,0,0), 0),
        gui.colorPoint(color(0,255,0), 0.5f),
        gui.colorPoint(color(0,0,255), 1)
);

adding a new color at runtime with the "stops" slider sets its position to around 0.5 - it should probably add it at the end, either copy or go further towards 1 than the last position on the existing list