Add support for mouse input transparency

This adds the GLFW_MOUSE_PASSTHROUGH window hint and attribute for
controlling whether mouse input passes through the window to whatever
window is behind it.

Fixes #1236.
Closes #1568.

src/null_window.c

@@ -362,6 +362,10 @@ void _glfwPlatformSetWindowFloating(_GLFWwindow* window, GLFWbool enabled)
     window->null.floating = enabled;
 }
 
+void _glfwPlatformSetWindowMousePassthrough(_GLFWwindow* window, GLFWbool enabled)
+{
+}
+
 float _glfwPlatformGetWindowOpacity(_GLFWwindow* window)
 {
     return window->null.opacity;