Add MGL entry points and port spinning_cube to use them

examples/spinning_cube/BUILD.gn

Index: examples/spinning_cube/BUILD.gn
diff --git a/examples/spinning_cube/BUILD.gn b/examples/spinning_cube/BUILD.gn
index 65b70595d260d21d777200415c0c78009f5a94b4..801e086e49993f8db2767b09e26d12391231604e 100644
--- a/examples/spinning_cube/BUILD.gn
+++ b/examples/spinning_cube/BUILD.gn
@@ -15,6 +15,8 @@ mojo_native_application("spinning_cube") {
     ":lib",
     "//base",
     "//mojo/public/c/gles2",
+    "//mojo/public/c/gpu",
+    "//mojo/public/c/gpu:gpu_onscreen",
     "//mojo/public/cpp/application:standalone",
     "//mojo/public/cpp/bindings",
     "//mojo/public/cpp/environment",
@@ -34,7 +36,7 @@ source_set("lib") {
   deps = [
     "//base",
     "//mojo/common",
-    "//mojo/public/c/gles2",
+    "//mojo/public/c/gpu",
     "//mojo/public/cpp/environment",
   ]
 }