dx9: Explicitly specify the backbuffer size.

Some people are getting black screens with Direct3D9 now, this is just a
theory.  Can revert if it doesn't help.

UI/NativeApp.cpp

@@ -713,9 +713,9 @@ void NativeRender() {
 		TakeScreenshot();
 	}
 
-	if (g_Config.iGPUBackend == GPU_BACKEND_DIRECT3D9) {
-		if (resized) {
-			resized = false;
+	if (resized) {
+		resized = false;
+		if (g_Config.iGPUBackend == GPU_BACKEND_DIRECT3D9) {
 #ifdef _WIN32
 			D3D9_Resize(0);
 #endif