fix: capture save thumbnails directly from D3D11 backbuffer

The new 4JLibs CaptureThumbnail produces all-black 64x64 PNGs because
its internal m_backBufferTexture copy fails silently. Bypass it entirely
and capture from the swap chain backbuffer using the same proven approach
as our F2 screenshot, with center-crop and downsample to 64x64 PNG.

Minecraft.Client/Windows64/Windows64_App.cpp

@@ -9,6 +9,11 @@
 #include "../../Minecraft.World/LevelSettings.h"
 #include "../../Minecraft.World/BiomeSource.h"
 #include "../../Minecraft.World/LevelType.h"
+#include "stb_image_write.h"
+
+extern ID3D11Device* g_pd3dDevice;
+extern ID3D11DeviceContext* g_pImmediateContext;
+extern IDXGISwapChain* g_pSwapChain;
 
 CConsoleMinecraftApp app;
 
@@ -33,9 +38,120 @@ void CConsoleMinecraftApp::FatalLoadError()
 {
 }
 
+static const int THUMBNAIL_SIZE = 64;
+
 void CConsoleMinecraftApp::CaptureSaveThumbnail()
 {
-	RenderManager.CaptureThumbnail(&m_ThumbnailBuffer);
+	if (!g_pSwapChain || !g_pd3dDevice || !g_pImmediateContext)
+		return;
+
+	// Release any previous capture
+	if (m_ThumbnailBuffer.Allocated())
+		m_ThumbnailBuffer.Release();
+
+	// Get the backbuffer
+	ID3D11Texture2D* pBackBuffer = nullptr;
+	HRESULT hr = g_pSwapChain->GetBuffer(0, __uuidof(ID3D11Texture2D), (void**)&pBackBuffer);
+	if (FAILED(hr))
+		return;
+
+	D3D11_TEXTURE2D_DESC backDesc = {};
+	pBackBuffer->GetDesc(&backDesc);
+
+	// Create a staging texture at backbuffer size to read pixels
+	D3D11_TEXTURE2D_DESC stagingDesc = backDesc;
+	stagingDesc.Usage = D3D11_USAGE_STAGING;
+	stagingDesc.BindFlags = 0;
+	stagingDesc.CPUAccessFlags = D3D11_CPU_ACCESS_READ;
+	stagingDesc.MiscFlags = 0;
+
+	ID3D11Texture2D* pStaging = nullptr;
+	hr = g_pd3dDevice->CreateTexture2D(&stagingDesc, nullptr, &pStaging);
+	if (FAILED(hr))
+	{
+		pBackBuffer->Release();
+		return;
+	}
+
+	g_pImmediateContext->CopyResource(pStaging, pBackBuffer);
+	pBackBuffer->Release();
+
+	D3D11_MAPPED_SUBRESOURCE mapped = {};
+	hr = g_pImmediateContext->Map(pStaging, 0, D3D11_MAP_READ, 0, &mapped);
+	if (FAILED(hr))
+	{
+		pStaging->Release();
+		return;
+	}
+
+	// Downsample to THUMBNAIL_SIZE x THUMBNAIL_SIZE with simple box filter
+	unsigned char* thumb = new unsigned char[THUMBNAIL_SIZE * THUMBNAIL_SIZE * 4];
+
+	// Crop to square (center crop), then scale down
+	UINT srcSize = (backDesc.Width < backDesc.Height) ? backDesc.Width : backDesc.Height;
+	UINT offsetX = (backDesc.Width - srcSize) / 2;
+	UINT offsetY = (backDesc.Height - srcSize) / 2;
+
+	for (int ty = 0; ty < THUMBNAIL_SIZE; ty++)
+	{
+		for (int tx = 0; tx < THUMBNAIL_SIZE; tx++)
+		{
+			// Map thumbnail pixel to source region
+			UINT sx = offsetX + (tx * srcSize) / THUMBNAIL_SIZE;
+			UINT sy = offsetY + (ty * srcSize) / THUMBNAIL_SIZE;
+
+			const unsigned char* src = (const unsigned char*)mapped.pData + sy * mapped.RowPitch + sx * 4;
+			unsigned char* dst = thumb + (ty * THUMBNAIL_SIZE + tx) * 4;
+
+			dst[0] = src[0]; // R (or B depending on format, but BGRA->RGBA swap below)
+			dst[1] = src[1]; // G
+			dst[2] = src[2]; // B
+			dst[3] = 0xFF;   // A
+		}
+	}
+
+	g_pImmediateContext->Unmap(pStaging, 0);
+	pStaging->Release();
+
+	// If backbuffer is BGRA, swap to RGBA for PNG
+	if (backDesc.Format == DXGI_FORMAT_B8G8R8A8_UNORM || backDesc.Format == DXGI_FORMAT_B8G8R8A8_UNORM_SRGB)
+	{
+		for (int i = 0; i < THUMBNAIL_SIZE * THUMBNAIL_SIZE; i++)
+		{
+			unsigned char tmp = thumb[i * 4];
+			thumb[i * 4] = thumb[i * 4 + 2];
+			thumb[i * 4 + 2] = tmp;
+		}
+	}
+
+	// Encode to PNG in memory using stbi_write_png_to_func
+	struct PngBuffer { unsigned char* data; int size; int capacity; } pngBuf = {};
+	pngBuf.capacity = THUMBNAIL_SIZE * THUMBNAIL_SIZE * 4 + 256;
+	pngBuf.data = (unsigned char*)malloc(pngBuf.capacity);
+	pngBuf.size = 0;
+
+	stbi_write_png_to_func([](void* ctx, void* data, int size) {
+		PngBuffer* buf = (PngBuffer*)ctx;
+		if (buf->size + size > buf->capacity)
+		{
+			buf->capacity = (buf->size + size) * 2;
+			buf->data = (unsigned char*)realloc(buf->data, buf->capacity);
+		}
+		memcpy(buf->data + buf->size, data, size);
+		buf->size += size;
+	}, &pngBuf, THUMBNAIL_SIZE, THUMBNAIL_SIZE, 4, thumb, THUMBNAIL_SIZE * 4);
+	delete[] thumb;
+
+	if (pngBuf.size > 0)
+	{
+		m_ThumbnailBuffer.m_type = ImageFileBuffer::e_typePNG;
+		m_ThumbnailBuffer.m_pBuffer = pngBuf.data;
+		m_ThumbnailBuffer.m_bufferSize = pngBuf.size;
+	}
+	else
+	{
+		free(pngBuf.data);
+	}
 }
 void CConsoleMinecraftApp::GetSaveThumbnail(PBYTE *pbData,DWORD *pdwSize)
 {