Graphics: Rework RenderTargets

- RenderTarget have been moved to the Graphics module and are now lightweight objects between the target of rendering (swapchain or texture)
- RenderTexture no longer require a blit between the framegraph texture and the target texture (the target texture is now directly rendered onto using a new feature of the framegraph)
- ForwardFramePipeline viewers are now properly ordered by render order

tests/GraphicsTest/main.cpp

@@ -76,7 +76,7 @@ int main()
 
 	Nz::Vector2ui windowSize = window.GetSize();
 
-	Nz::Camera camera(&windowSwapchain);
+	Nz::Camera camera(std::make_shared<Nz::RenderWindow>(windowSwapchain));
 	camera.UpdateClearColor(Nz::Color::Gray());
 
 	Nz::ViewerInstance& viewerInstance = camera.GetViewerInstance();