X11: Don't override glxSwapInterval function pointers loaded by GLAD

[akien-mga]

• Fixes Crash GLManager_X11::set_use_vsync(bool) #68722.
• Supersedes Manually load glXSwapInterval function pointers #68714.

Co-authored-by: @alcomposer

Tested on Intel HD 630 and AMD Radeon Vega M with Mesa 22.2.3.
And @alcomposer tested the same patch with Nvidia driver 520.56.06.

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BTW there's more stuff in this file that could likely be cleaned up now that we use GLAD2's glx wrapper.
E.g. these overrides seem to duplicate what's already in glad/glx.h:

#define GLX_CONTEXT_MAJOR_VERSION_ARB 0x2091
#define GLX_CONTEXT_MINOR_VERSION_ARB 0x2092

[Riteo]

My bad, this is my fault. This wasn't "an issue that has been around since the early GLES3 port" like @clayjohn said or a driver issue, this was the result of my GLX dynamic loading PR. I'm sorry.

I cleaned this code myself to use GLAD but I forgot to remove the local pointers, and not using X11 so much I didn't notice the crash.

I do agree that the whole GLManager_X11 class could be cleaned quite a bit and perhaps could be refactored to follow naming conventions more (starting from the class name). The EGLManager I wrote is pretty much a refactored/cleaned up version of this class ported to EGL.

[clayjohn]

This is indeed the proper fix. Thanks to @alcomposer for pointing that out!