[vk] Add missing flush per spec (#2624)

We copy pixels into a CPU-side staging buffer and then ask the GPU to read from it. On some systems those CPU writes aren’t automatically visible to the GPU unless explicitly flushed, so the GPU can sometimes read stale data. By calling buffer.Flush() immediately after writing, we force those CPU changes to become visible to the device, ensuring the GPU sees the latest frame. However, this is an emulator, so sometimes what spec says may not work cause reasons.

Co-authored-by: Ribbit <ribbit@placeholder.com>
Reviewed-on: #2624
Reviewed-by: Shinmegumi <shinmegumi@eden-emu.dev>
Reviewed-by: MaranBr <maranbr@eden-emu.dev>
Co-authored-by: Ribbit <ribbit@eden-emu.dev>
Co-committed-by: Ribbit <ribbit@eden-emu.dev>

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{