title: QLabel showing an image inside a QScrollArea
body: <p>I've successfully implemented a Image Viewer (for DICOM) in Qt.
  I can see the image and I can zoom in and out correctly.</p>
  
  <p>Now, I want to see scroll bars if the image is too big to show when I zoom in.</p>
  
  <p>I've used the UI. I placed a <strong>QScrollArea</strong>. Inside, the <strong>QLabel</strong>.
  The <em>verticalScrollBarPolicy</em> is <em>ScrollBarAsNeeded</em>.
  The <em>horizontalScrollBarPolicy</em> is <em>ScrollBarAsNeeded</em>.</p>
  
  <p>The problem is: it doesn't work. I zoom in, but no scrollbar appears.</p>
  
  <h1>Second try: using a layout inside the QScrollArea.</h1>
  
  <p>So now there's a <strong>QWidget</strong> between the QScrollArea and the QLabel: a horizontal layout.
  Opened the same image, now I can see a vertical scroll bar on the right. The image is streched from left to right. When I zoom the image gets its correct proportion.</p>
  
  <p>BUT... I zoom out and the scroll bar is the same, even if I can see the whole image. The horizontal scroll bar never appears.</p>
  
  <p>Resizing the QLabel doesn't seem to affect. But if I resize the QScrollArea (resizing the main window) the horizontal scroll bar appears.</p>
  
  <h1>I've been checking some numbers:</h1>
  
  <h2>In the QScrollArea</h2>
  
  <ul>
  <li>Its size changes: below 599 width (why this number? I can't see it anywhere) the horizontal bar appears.</li>
  <li><em>sizeHint()</em> always returns the same values: 33x41</li>
  </ul>
  
  <h2>In the QLabel</h2>
  
  <ul>
  <li>The dimensions change, but that doesn't affect.</li>
  <li><em>sizeHint()</em> always returns the same values: 560x1558</li>
  </ul>
  
  <p>Here is the code in XML from the UI designer:</p>
  
  <pre><code>&lt;widget class="QWidget" name="centralWidget"&gt;
     &lt;property name="autoFillBackground"&gt;
      &lt;bool&gt;false&lt;/bool&gt;
     &lt;/property&gt;
     &lt;layout class="QVBoxLayout" name="verticalLayout"&gt;
      &lt;property name="margin"&gt;
       &lt;number&gt;0&lt;/number&gt;
      &lt;/property&gt;
      &lt;item&gt;
       &lt;widget class="QScrollArea" name="scrollArea"&gt;
        &lt;property name="widgetResizable"&gt;
         &lt;bool&gt;true&lt;/bool&gt;
        &lt;/property&gt;
        &lt;property name="alignment"&gt;
         &lt;set&gt;Qt::AlignLeading|Qt::AlignLeft|Qt::AlignTop&lt;/set&gt;
        &lt;/property&gt;
        &lt;widget class="QWidget" name="scrollAreaWidgetContents"&gt;
         &lt;property name="geometry"&gt;
          &lt;rect&gt;
           &lt;x&gt;0&lt;/x&gt;
           &lt;y&gt;0&lt;/y&gt;
           &lt;width&gt;637&lt;/width&gt;
           &lt;height&gt;649&lt;/height&gt;
          &lt;/rect&gt;
         &lt;/property&gt;
         &lt;layout class="QHBoxLayout" name="horizontalLayout_2"&gt;
          &lt;item&gt;
           &lt;widget class="QLabel" name="miImagen"&gt;
            &lt;property name="sizePolicy"&gt;
             &lt;sizepolicy hsizetype="Preferred" vsizetype="Preferred"&gt;
              &lt;horstretch&gt;0&lt;/horstretch&gt;
              &lt;verstretch&gt;0&lt;/verstretch&gt;
             &lt;/sizepolicy&gt;
            &lt;/property&gt;
            &lt;property name="autoFillBackground"&gt;
             &lt;bool&gt;true&lt;/bool&gt;
            &lt;/property&gt;
            &lt;property name="scaledContents"&gt;
             &lt;bool&gt;true&lt;/bool&gt;
            &lt;/property&gt;
            &lt;property name="alignment"&gt;
             &lt;set&gt;Qt::AlignLeading|Qt::AlignLeft|Qt::AlignTop&lt;/set&gt;
            &lt;/property&gt;
           &lt;/widget&gt;
          &lt;/item&gt;
         &lt;/layout&gt;
        &lt;/widget&gt;
       &lt;/widget&gt;
      &lt;/item&gt;
     &lt;/layout&gt;
    &lt;/widget&gt;
  </code></pre>
  
  <p>What am I missing? Thank you.</p>