ok: basic Vias

Not sure if these simple Src/Dst interfaces will last.
Vias are a little tricky, and some may be impossible.

Change-Id: I42d19b1ee74b51a830bb781f25a888c0b32ba98c
Reviewed-on: https://skia-review.googlesource.com/10174
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index ed8b07b..b38f003 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1154,6 +1154,7 @@
       "tools/ok.cpp",
       "tools/ok_dsts.cpp",
       "tools/ok_srcs.cpp",
+      "tools/ok_vias.cpp",
     ]
     deps = [
       ":gm",