Apple is deprecating OpenGL on iOS 12, which means in longer term we may need to reimplement our renderer stack again O_o

OpenGL should still be working on iOS for quite a long time though, so I don't think this is a high priority work.

Vulkan is generally the successor of OpenGL, but Apple decides not to support it in favor of its proprietary metal API. So here are a few options we have for now:

* Find and use some OpenGL-to-Metal mapping API and call it a day.
* Use some higher level graphic APIs like Skia (which is in progress of supporting Metal).
* Rewrite the renderer in Vulkan and use some Vulkan-to-Metal mapping API.