softgpu: Fix s8 primitives in throughmode.

Also always cull no-position verts, hardware too.  Matches tests.

test.py

@@ -158,6 +158,7 @@ tests_good = [
   "gpu/ge/enqueueparam",
   "gpu/ge/queue",
   "gpu/primitives/indices",
+  "gpu/primitives/invalidprim",
   "gpu/primitives/trianglefan",
   "gpu/primitives/trianglestrip",
   "gpu/primitives/triangles",
@@ -400,7 +401,6 @@ tests_next = [
   "gpu/primitives/bezier",
   "gpu/primitives/continue",
   "gpu/primitives/immediate",
-  "gpu/primitives/invalidprim",
   "gpu/primitives/lines",
   "gpu/primitives/linestrip",
   "gpu/primitives/points",