update slides

BUG=skia:

Review URL: https://codereview.chromium.org/686853005
diff --git a/src/utils/SkLua.cpp b/src/utils/SkLua.cpp
index 967233d..b12e3e9 100644
--- a/src/utils/SkLua.cpp
+++ b/src/utils/SkLua.cpp
@@ -1906,7 +1906,7 @@
 }
 
 static int lsk_newRasterSurface(lua_State* L) {
-    int width = lua2int_def(L, 2, 0);
+    int width = lua2int_def(L, 1, 0);
     int height = lua2int_def(L, 2, 0);
     SkImageInfo info = SkImageInfo::MakeN32Premul(width, height);
     SkSurface* surface = SkSurface::NewRaster(info);