Add glfwGetWindowOpacity and glfwSetWindowOpacity

This adds support for setting the opacity of the whole window, including
any decorations.

Fixes #1089.

src/mir_window.c

@@ -648,6 +648,15 @@ void _glfwPlatformSetWindowFloating(_GLFWwindow* window, GLFWbool enabled)
                     "Mir: Unsupported function %s", __PRETTY_FUNCTION__);
 }
 
+float _glfwPlatformGetWindowOpacity(_GLFWwindow* window)
+{
+    return 1.f;
+}
+
+void _glfwPlatformSetWindowOpacity(_GLFWwindow* window, float opacity)
+{
+}
+
 void _glfwPlatformPollEvents(void)
 {
     EventNode* node = NULL;