glx: Support making a context current without surface

[mbernat]

This mirrors the EGL behavior and follows in footsteps of #1710.

This change should be valid according to https://registry.khronos.org/OpenGL/extensions/ARB/GLX_ARB_create_context.txt (emphasis mine)

"If either <draw> or <read> are not a valid GLX drawable, a
GLXBadDrawable error is generated, unless draw and read are both
None and the OpenGL version supported by <ctx> is 3.0 or greater. In
this case the context is made current without a default framebuffer,
as defined in chapter 4 of the OpenGL 3.0 Specification."

• 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

[MarijnS95]

I presume this comes from the suggestion at #1710 (comment)?

[mbernat]

@MarijnS95 Yes, indeed.

Didn't mean to open the PR just yet, hence the stump form of the PR request, but I'd be happy to see it through properly.

[MarijnS95]

@mbernat no worries, just figured we should update the title to include the without surface or surfaceless keyword :)

Happy that you included the documentation that clarifies that this is allowed, because glxMakeContextCurrent() does not (as expected).

Like the rest of glutin, how about we add some extra version/extension validation before calling the function like this, and have appropriate error handling otherwise?

[mbernat]

@MarijnS95 Let me know whether the validation I added is along the lines you had in mind.

I also implemented the API for WGL elsewhere but the validation is harder to do because its Display doesn't carry a version info, unlike in the other backends. Also, I don't really have a windows use case to test this against. But other than that I can post what I have so far as a separate PR and we can take it from there.

[mbernat]

I have fixed the validation, the previous commit was using the GLX version stored in Display rather than the OpenGL version as determined by context attributes and profiles (oops).

It's a bit hairy now since the version is only computed in the middle of the context creation and I needed to get the information about surfaceless support out from there; it would be better to compute the version much earlier. Let me know and I'd be happy to try refactoring it.

Other than that, this should be ready for review.

@MarijnS95 @kchibisov please take a look whenever it's convenient for you 🙏
Not urgent but I'm not very experienced with open source and the review process here, so just a gentle reminder 😇

[strohel]

I've just successfully tested this one (in a proprietary application).