draw: Add flags to draw_prim_info.

draw: Add flags to draw_prim_info.

A primitive may be splitted in frontends.  The splitted primitives
should convey certain flag bits so that the decomposer can correctly
decide the stipple or edge flags.

This commit adds flags to draw_prim_info and updates the decomposer to
honor the flags.  Frontends and middle ends will be updated later.