Add MacOS (Cgl) make_current_surfaceless() implementation

[bschwind]

• Tested on all platforms changed
• Added an entry to CHANGELOG.md if knowledge of this change could be valuable to users
• Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• Created or updated an example program if it would help users understand this functionality

I have tested this on a complex OpenGL application which uses multiple "surfaceless" contexts on many threads while sharing textures and having those textures displayed on the main thread with a windowed OpenGL context, and everything appears to function correctly.

[kchibisov]

--

next time would suggest to re-request review (ui on the right).

[bschwind]

next time would suggest to re-request review (ui on the right).

Roger that!

[kchibisov]

There's also the same APIs for glx/wgl from what I can tell, so we may have it at top-level at some point.

it's just they both need extensions, so display feature may be needed for users to configure their pipeline beforehand.

[bschwind]

There's also the same APIs for glx/wgl from what I can tell, so we may have it at top-level at some point.

Yep, a colleague of mine has added similar functionality for glx, so we'll probably have another PR up for that soon. I currently don't have a windows machine so I can't test wgl at the moment, but hopefully I'll get one soon.

[kchibisov]

wgl should be the same as glx since they were designed by the same process.