Fixes #559

On macOS, if you are using OpenGL, motion stutters. Motion is not smooth/fluent. You can see it immediately when you run

cargo run --example quad

The problem is that there isn't any real frame pacing in the current main loop. The main loop pushes out a new frame each iteration with flushBuffer and relies on VSYNC (swap_interval = 1) for pacing. But that's not enough on macOS. flushBuffer gives us the new frame, but it's up to the display server (Quartz Compositor) to decide when to use it.

This change adds a frame pacer using macOS's CVDisplayLink callback. Basically what this change does is this:

• Adds a flag frame_ready that tells us when to start rendering the next frame.
• CVDisplayLink callback sets the flag to true
• The main loop wait for frame_ready to become true and continues with the next iteration.

There might be other ways to do it. But I wanted to keep the main loop intact as much as possible.

With this frame pacer motion is now super smooth 😄

Before version v0.4.6 it was still reasonably smooth. That's because rendering was done differently. In v.0.4.5 each iteration of the main loop called setNeedsDisaply:YES on the view to trigger a redraw. This marks the view as 'dirty' signaling the display server to update it. The redraw performed a flushBuffer with VSYNC.