Add MGL entry points and port spinning_cube to use them

mojo/gles2/BUILD.gn

Index: mojo/gles2/BUILD.gn
diff --git a/mojo/gles2/BUILD.gn b/mojo/gles2/BUILD.gn
index 92b2375501f5710f5c644c0069450ab86df48ade..529a25f198957f0873934a85d232cc42f32299b7 100644
--- a/mojo/gles2/BUILD.gn
+++ b/mojo/gles2/BUILD.gn
@@ -16,9 +16,9 @@ source_set("gles2") {
   sources = [
     "command_buffer_client_impl.cc",
     "command_buffer_client_impl.h",
-    "gles2_impl.cc",
     "gles2_context.cc",
     "gles2_context.h",
+    "gles2_impl.cc",
   ]
 
   defines = [
@@ -50,3 +50,19 @@ source_set("gles2") {
     "//services/gles2:lib",
   ]
 }
+
+source_set("mgl") {
+  sources = [
+    "mgl_impl.cc",
+  ]
+
+  configs += [ ":gles2_use_mojo" ]
+
+  deps = [
+    ":gles2",
+    "//mojo/public/c/gles2:headers",
+    "//mojo/public/c/gpu:MGL",
+    "//mojo/public/c/gpu:MGL_onscreen",
+    "//mojo/public/cpp/system",
+  ]
+}