Improve OSX+Metal4 backends and examples

The current osx+metal4 backends have some issues that this PR is trying to address.

1. Embed viewport flickering and other glitches when detaching another viewport

• Reused in-flight vertex buffer. When a torn-off window drew right after the main window in the same frame, the buffer pool handed it the main window's buffer — still being read by the GPU — and it wrote its own vertices over it → the main/embedded windows blinked garbage. Fix: used buffers are held aside and only reused after the GPU is done (never within the same frame). Backed by proper frame-slot rotation + a small "frames in flight" gate so it's safe across frames too.

https://github.com/user-attachments/assets/bc7ae67c-9ff5-4511-9201-a4097a8c992e

https://github.com/user-attachments/assets/36f687c2-8395-4c80-98e6-5e09c18da446

2. Window content shuttering when resizing from bottom/right corner The fix is partly based on https://thume.ca/2019/06/19/glitchless-metal-window-resizing/ but had to be adapted for Metal 4

• Change the MTKView to custom CAMetalLayer view, DuringViewResize policy, drawableSize in setFrameSize:, CVDisplayLink
• Metal 4 has no [commandBuffer waitUntilScheduled]
• Setting presentsWithTransaction = YES always (as the post does) froze the Metal 4 window — the CVDisplayLink-driven layer starved the drawable pool. So metal4 keeps it NO, and turns it on only during a live resize (layer.presentsWithTransaction = self.metalView.inLiveResize), presents the canonical Metal 4 way (waitForDrawable/commit/signalDrawable/present, no waitUntilScheduled), and during a resize lets AppKit drive the redraw (the display link steps aside) so the transaction commits.

Frankly, I'm open to a cleaner solution if there's one but this is working.

https://github.com/user-attachments/assets/87ed87dc-b10c-4f26-95ea-c342f3d36c9d

I took the opportunity to add some polishing for :

1. The pop animation on tear down and reduced the one frame blink occurence

https://github.com/user-attachments/assets/598d05d8-6f23-43a9-8af5-787f17a28561

2. Made viewports to always stay on top of the app and respect other apps focus/defocus
3. Added Idle throttle for less cpu consumption
4. unlocked refresh rate