Unlimited mouse button input mode

This adds the GLFW_UNLIMITED_MOUSE_BUTTONS input mode which permits
mouse buttons over GLFW_MOUSE_BUTTON_LAST to be reported to the mouse
button callback.

Closes #2423

tests/events.c

@@ -630,6 +630,7 @@ int main(int argc, char** argv)
             glfwTerminate();
             exit(EXIT_FAILURE);
         }
+        glfwSetInputMode(slots[i].window, GLFW_UNLIMITED_MOUSE_BUTTONS, GLFW_TRUE);
 
         glfwSetWindowUserPointer(slots[i].window, slots + i);
 

tests/window.c

@@ -78,6 +78,7 @@ int main(int argc, char** argv)
         glfwTerminate();
         exit(EXIT_FAILURE);
     }
+    glfwSetInputMode(window, GLFW_UNLIMITED_MOUSE_BUTTONS, GLFW_TRUE);
 
     glfwMakeContextCurrent(window);
     gladLoadGL(glfwGetProcAddress);