softpipe: rework to use the llvmpipe winsys

Promote the llvmpipe winsys more or less unchanged to
state_trackers/sw_winsys.h.

Some minor breakages:
  - softpipe::texture_blanket is broken, but scheduled for removal anyway.
  - haven't fixed up g3vdl yet.

src/gallium/drivers/softpipe/SConscript

@@ -7,6 +7,7 @@ softpipe = env.ConvenienceLibrary(
 	source = [
 		'sp_fs_exec.c',
 		'sp_fs_sse.c',
+		'sp_buffer.c',
 		'sp_clear.c',
 		'sp_context.c',
 		'sp_draw_arrays.c',
@@ -33,8 +34,7 @@ softpipe = env.ConvenienceLibrary(
 		'sp_tex_tile_cache.c',
 		'sp_texture.c',
 		'sp_tile_cache.c',
-		'sp_video_context.c',
-		'sp_winsys.c'
+		'sp_video_context.c'
 	])
 
 Export('softpipe')