Native swapchain on Mac/GL with a custom layer

[kvark]

@mstange got a CA layer implemented for GL that manages and presents IOSurface objects. We could technically get this as a WSI layer on Mac/GL that has little friction with Vulkan API (unlike the regular GL framebuffer stuff).

https://github.com/mstange/core-animation-opengl-two-windows

@zegentzy any thoughts on that? Maybe you've already got this covered, I just don't know the current state of GL backend very well.

[goddessfreya]

I got no familiarity with OSX. Know nothing about this.

[kvark]

I'm thinking of dropping macOS support from OpenGL completely, see #3468