SDF Squircle

Inputs
float Radius | default 0.5
float Roundness | default 0.2
Behaviour
Outputs the signed distance field for a Squircle, based on the provided Radius. Roundness determines how much the corners are rounded but keeps the sdf radius the same. Roundness is only well defined between 0- 1, values above 1 lead to collapse of the corners to the inside.
