app: call Start on darwin

Still working on how/when to call Stop.

Change-Id: Ice56c4c71ba351765657bb86dc01649c8238859e
Reviewed-on: https://go-review.googlesource.com/1885
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

app/darwin.go

@@ -74,6 +74,9 @@ func initGL() {
 	var id C.GLuint
 	C.glGenVertexArrays(1, &id)
 	C.glBindVertexArray(id)
+	if cb.Start != nil {
+		cb.Start()
+	}
 }
 
 var cb Callbacks