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

include/Nazara/Graphics/FrameGraphStructs.hpp

@@ -16,6 +16,8 @@
 
 namespace Nz
 {
+	class Texture;
+
 	struct FrameGraphTextureData
 	{
 		struct ViewData
@@ -25,6 +27,7 @@ namespace Nz
 		};
 
 		std::optional<ViewData> viewData;
+		std::shared_ptr<Texture> externalTexture;
 		std::string name;
 		ImageType type;
 		PixelFormat format;