[MNT]: macOS backend likely double-draws renderer output #31875

Summary

Right now, -[View drawRect:] does the following:

1. Acquires the GIL.
2. Calls get_renderer() to get the agg renderer
3. Calls buffer_rgba() to get a memoryview of the buffer.
4. Performs a malloc() and copies the contents to the new buffer.
5. Wraps in CGDataProviderRef, then CGImageRef
6. Draws into the View's buffer via CGContextDrawImage()
7. View's buffer is then composited to the window's buffer.
8. Releases the GIL.

We can probably merge Step 6 / Step 7 by using a layer-backed view and setting CALayer.contents directly. There may be more optimizations here as well, if we can directly "push" a buffer to the view rather than relying on the -drawRect: call during the render cycle.

In either case, the rubberband would need to be a separate layer or view (non-opaque).

Proposed fix

No response