Add glfwGetGLXFBConfig native access function

This adds the glfwGetGLXFBConfig function for querying the GLXFBConfig
the GLXWindow of a window.

This commit is a squashed and modified version of PR #1925 by knokko.
The following changes were made by elmindreda:

The function signature was changed to handle GLXFBConfig being an opaque
value in core GLX.  The function error checks were fixed and updated.
The struct member name was changed.  The struct member clearing on
context destruction was removed.  All documentation snippets were
updated.

Closes #1925

src/glx_context.c

@@ -626,6 +626,8 @@ GLFWbool _glfwCreateContextGLX(_GLFWwindow* window,
         return GLFW_FALSE;
     }
 
+    window->context.glx.fbconfig = native;
+
     window->context.makeCurrent = makeContextCurrentGLX;
     window->context.swapBuffers = swapBuffersGLX;
     window->context.swapInterval = swapIntervalGLX;
@@ -719,5 +721,29 @@ GLFWAPI GLXWindow glfwGetGLXWindow(GLFWwindow* handle)
     return window->context.glx.window;
 }
 
+GLFWAPI int glfwGetGLXFBConfig(GLFWwindow* handle, GLXFBConfig* config)
+{
+    _GLFW_REQUIRE_INIT_OR_RETURN(GLFW_FALSE);
+
+    if (_glfw.platform.platformID != GLFW_PLATFORM_X11)
+    {
+        _glfwInputError(GLFW_PLATFORM_UNAVAILABLE, "GLX: Platform not initialized");
+        return GLFW_FALSE;
+    }
+
+    _GLFWwindow* window = (_GLFWwindow*) handle;
+    assert(window != NULL);
+    assert(config != NULL);
+
+    if (window->context.source != GLFW_NATIVE_CONTEXT_API)
+    {
+        _glfwInputError(GLFW_NO_WINDOW_CONTEXT, NULL);
+        return GLFW_FALSE;
+    }
+
+    *config = window->context.glx.fbconfig;
+    return GLFW_TRUE;
+}
+
 #endif // _GLFW_X11
 

src/x11_platform.h

@@ -470,6 +470,7 @@ typedef struct _GLFWcontextGLX
 {
     GLXContext      handle;
     GLXWindow       window;
+    GLXFBConfig     fbconfig;
 } _GLFWcontextGLX;
 
 // GLX-specific global data