Implement sprite portals

src/deh_tables.c

src/doomdef.h

src/r_context.h

src/r_defs.h

src/r_draw.h

src/r_draw8.c

src/r_draw8_npo2.c

src/r_main.c

src/r_main.h

src/r_plane.c

src/r_portal.h

src/r_segs.c

src/r_state.h

src/r_things.c

src/r_things.h