Fixed X11 mouse button mismatch.

Bug introduced by 4d146cb39d.

Closes #449.

README.md

@@ -147,6 +147,7 @@ skills.
  - Hans Mackowiak
  - Kyle McDonald
  - David Medlock
+ - Bryce Mehring
  - Jonathan Mercier
  - Marcel Metz
  - Jonathan Miller

src/x11_window.c

@@ -1049,7 +1049,7 @@ static void processEvent(XEvent *event)
                 // Additional buttons after 7 are treated as regular buttons
                 // We subtract 4 to fill the gap left by scroll input above
                 _glfwInputMouseClick(window,
-                                     event->xbutton.button - 4,
+                                     event->xbutton.button - Button1 - 4,
                                      GLFW_RELEASE,
                                      mods);
             }