Default arrow head with antialiasing

Since the argument of drawLine is image.Point (a structure with x,y as integers), the floating point number is cast to an integer.

https://github.com/awslabs/diagram-as-code/blob/cc6f6ce118fab8231a735216cc0766cd069d0f4c/internal/types/link.go#L116-L119

Considering extensibility, an option is to create a Point structure with x and y as floating point numbers.

Start with internal/types/link.go around lines 116–119 and inspect how the drawLine arguments convert floating-point coordinates to image.Point integers. Determine the intended antialiasing behavior for the default arrow head, then verify that the resulting links render smoothly without changing their geometry.

Written by the indexing model from the issue text.