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

src/internal.h

@@ -544,6 +544,7 @@ struct _GLFWwindow
     GLFWbool            stickyKeys;
     GLFWbool            stickyMouseButtons;
     GLFWbool            lockKeyMods;
+    GLFWbool            disableMouseButtonLimit;
     int                 cursorMode;
     char                mouseButtons[GLFW_MOUSE_BUTTON_LAST + 1];
     char                keys[GLFW_KEY_LAST + 1];