Add getClipStack method to lua SkCanvas

tools/lua/dump_clipstack_at_restore.lua

+function sk_scrape_startcanvas(c, fileName)
+    canvas = c
+    clipstack = {}
+    restoreCount = 0
+end
+
+function sk_scrape_endcanvas(c, fileName)
+    canvas = nil
+end
+
+function sk_scrape_accumulate(t)
+    if (t.verb == "restore") then
+        restoreCount = restoreCount + 1;
+        io.write("Clip Stack at restore #", restoreCount, ":\n")
+        for i = 1, #clipstack do
+            local element = clipstack[i];
+            io.write("\t", element["op"], ", ", element["type"], ", aa:", tostring(element["aa"]), "\n")
+        end
+        io.write("\n")
+    else
+        clipstack = canvas:getClipStack()
+    end
+end
+
+function sk_scrape_summarize() end