[vk] Add missing flush per spec

src/video_core/renderer_vulkan/present/layer.cpp

@@ -280,6 +280,7 @@ void Layer::UpdateRawImage(const Tegra::FramebufferConfig& framebuffer, size_t i
         Tegra::Texture::UnswizzleTexture(
             mapped_span.subspan(image_offset, linear_size), std::span(host_ptr, tiled_size),
             bytes_per_pixel, framebuffer.width, framebuffer.height, 1, block_height_log2, 0);
+        buffer.Flush();  // Ensure host writes are visible before the GPU copy.
     }
 
     const VkBufferImageCopy copy{