v3d: Add a lowering pass to implement line smoothing

Adds a NIR lowering pass which implements line smoothing. The V3D hardware has a magic builtin varying that gives a coordinate along the axis perpendicular to the line. The pass uses this to determine whether a fragment on the line is outside the original line width and adjust the alpha component as needed. The patch also increases the line width that gets sent to the hardware in order to have some extra fragments to make transparent.