Default arrow head with antialiasing

[ugwis]

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.