Add glfwGetWindowOpacity and glfwSetWindowOpacity

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

Fixes #1089.

src/wl_window.c

@@ -692,6 +692,15 @@ void _glfwPlatformSetWindowFloating(_GLFWwindow* window, GLFWbool enabled)
                     "Wayland: Window attribute setting not implemented yet");
 }
 
+float _glfwPlatformGetWindowOpacity(_GLFWwindow* window)
+{
+    return 1.f;
+}
+
+void _glfwPlatformSetWindowOpacity(_GLFWwindow* window, float opacity)
+{
+}
+
 void _glfwPlatformPollEvents(void)
 {
     handleEvents(0);