title: Can't save image to file properly with cimg library
body: <p>Here is the entirety of my code:</p>
  
  <pre><code>#include "CImg.h"
  #include &lt;iostream&gt;
  
  using namespace cimg_library;
  int main() {
  
  CImg&lt;float&gt; image(100,100,1,3,0);
  const float color[] = {1.0,1.0,0.0};
  image.draw_point(50,50,color);
  image.save("file.bmp");
  CImgDisplay local(image, "Hah");
  while (true) {
      local.wait();
      }   
  
  }
  </code></pre>
  
  <p>This successfully displays what I expect in a window, namely, a completely black square
  with a white pixel at 50,50. However, file.bmp is simply a black square, without that
  pixel (and saving a cimg Image that has been modified with repeated calls to draw_point
  in a larger program that does something useful also fails). What's going on here?</p>