绘制带Alpha值的bgra数据

方法一，用AlphaBlend

        BITMAPINFO bmi;
        ZeroMemory(&bmi, sizeof(BITMAPINFO));

        // setup bitmap info  
        bmi.bmiHeader.biSize = sizeof(BITMAPINFOHEADER);
        bmi.bmiHeader.biWidth = bmp_width;
        bmi.bmiHeader.biHeight = -bmp_height;
        bmi.bmiHeader.biPlanes = 1;
        bmi.bmiHeader.biBitCount = 32;
        bmi.bmiHeader.biCompression = BI_RGB;
        bmi.bmiHeader.biSizeImage = bmp_width * bmp_height * 4;

        // create a DC for our bitmap -- the source DC for AlphaBlend  
        HDC dc_tmp = CreateCompatibleDC(dc);

        // create our DIB section and select the bitmap into the dc 
        VOID* pvBits;          // pointer to DIB section 
        HBITMAP hbitmap = CreateDIBSection(dc_tmp, &bmi, DIB_RGB_COLORS, &pvBits, NULL, 0x0);
        SelectObject(dc_tmp, hbitmap);

        // copy bitmap data into hbitmap
        memcpy(pvBits, bmp_buf, bmi.bmiHeader.biSizeImage);

        BLENDFUNCTION bf;      // structure for alpha blending 
        bf.BlendOp = AC_SRC_OVER;
        bf.BlendFlags = 0;
        bf.AlphaFormat = AC_SRC_ALPHA;   // use source alpha  
        bf.SourceConstantAlpha = 0xff;

        auto ret = AlphaBlend(dc, 
            dst_x,
            dst_y,
            bmp_width,
            bmp_height,
            dc_tmp,
            0,
            0,
            bmp_width,
            bmp_height,
            bf
        );

        // do cleanup 
        DeleteObject(hbitmap);
        DeleteDC(dc_tmp);

方法二，使用GDI+

        std::unique_ptr graphics(Gdiplus::Graphics::FromHDC(dc));
        std::unique_ptr bitmap(new Gdiplus::Bitmap(
            bmp_width,
            bmp_height,
            bmp_width * 4,
            PixelFormat32bppARGB,
            bmp_buf
        ));

        graphics->DrawImage(bitmap.get(), dst_x, dst_y);

GDI+ 要记得初始化先，否则graphics对象无法初始化

    // 开始GDI+
    Gdiplus::GdiplusStartupInput gdiplus_startup_input;
    Gdiplus::GdiplusStartup(&gdiplus_token_, &gdiplus_startup_input, NULL);
    ...
    // 结束GDI+
    Gdiplus::GdiplusShutdown(gdiplus_token_);