GLX: Add support for loading glvnd libGLX

(cherry picked from commit c18851f52e)

README.md

@@ -126,6 +126,7 @@ information on what to include when reporting a bug.
  - [Wayland] Bugfix: Connecting a mouse after `glfwInit` would segfault (#1450)
  - [Linux] Bugfix: Joysticks without buttons were ignored (#2042,#2043)
    [EGL] Added loading of glvnd `libOpenGL.so.0` where available for OpenGL
+ - [GLX] Added loading of glvnd `libGLX.so.0` where available
 
 
 ## Contact

src/glx_context.c

@@ -226,7 +226,10 @@ static GLFWglproc getProcAddressGLX(const char* procname)
     else if (_glfw.glx.GetProcAddressARB)
         return _glfw.glx.GetProcAddressARB((const GLubyte*) procname);
     else
+    {
+        // NOTE: glvnd provides GLX 1.4, so this can only happen with libGL
         return _glfw_dlsym(_glfw.glx.handle, procname);
+    }
 }
 
 static void destroyContextGLX(_GLFWwindow* window)
@@ -263,6 +266,7 @@ GLFWbool _glfwInitGLX(void)
 #elif defined(__OpenBSD__) || defined(__NetBSD__)
         "libGL.so",
 #else
+        "libGLX.so.0",
         "libGL.so.1",
         "libGL.so",
 #endif

src/glx_context.h

@@ -127,7 +127,6 @@ typedef struct _GLFWlibraryGLX
     int             eventBase;
     int             errorBase;
 
-    // dlopen handle for libGL.so.1
     void*           handle;
 
     // GLX 1.3 functions