Added GLFW OpenGL 2 backend

[aidenfoxx]

This is basically only needed to make zgui compatible with fixed function OpenGL in MacOS, which doesn't support compatibility profiles in OpenGL 3.

The backend is self-contained since it only uses GL 1.1 entry points; it links the system GL library directly rather than resolving symbols through zopengl, which officially only targets modern core profiles. As a result, the ImGui backend code is a direct copy from ImGui v1.92.1 (currently used by the zgui).

Notes:

• Consumers who want fixed function GL can import the system header directly via translate-c (example below).
• Consumers who also link zopengl on Windows should pass .export_linkage = .weak, to the dependency, otherwise GL 1.x symbols collide.

build.zig:

    const gl_c = b.addTranslateC(.{
        .root_source_file = b.path("src/gl.h"),
        .target = target,
        .optimize = optimize,
    });
    const gl_mod = gl_c.createModule();
    exe.root_module.addImport("gl", gl_mod);

src/gl.h

// Taken from `imgui_impl_opengl2.cpp` 
#if defined(_WIN32) && !defined(APIENTRY)
#define APIENTRY __stdcall
#endif
#if defined(_WIN32) && !defined(WINGDIAPI)
#define WINGDIAPI __declspec(dllimport)
#endif
#if defined(__APPLE__)
#define GL_SILENCE_DEPRECATION
#include <OpenGL/gl.h>
#else
#include <GL/gl.h>
#endif