GPUDevice: Add GPUDownloadTexture

Which can also be based in host/client memory.
Use it for screenshots and VRAM downloads.

src/core/gpu_hw.h

@@ -216,6 +216,7 @@ private:
   std::unique_ptr<GPUTexture> m_vram_depth_texture;
   std::unique_ptr<GPUTexture> m_vram_read_texture;
   std::unique_ptr<GPUTexture> m_vram_readback_texture;
+  std::unique_ptr<GPUDownloadTexture> m_vram_readback_download_texture;
   std::unique_ptr<GPUTexture> m_vram_replacement_texture;
   std::unique_ptr<GPUTexture> m_display_private_texture; // TODO: Move to base.