title: QPixmaps in PyQt don't get cleaned up properly when made directly from file and not from QImage
body: <p>Consider the following code:</p>
  
  <pre><code>from PyQt4 import QtCore, QtGui, QtNetwork, QtWebKit
  
  
  class ImageView(QtGui.QGraphicsView):
  
      def __init__(self, file):
          super().__init__()
          self.setScene(QtGui.QGraphicsScene())
          f = QtGui.QPixmap(QtGui.QImage(file))
          # f = QtGui.QPixmap(file)
          item = QtGui.QGraphicsPixmapItem(f)
          self.scene().addItem(item)
  
      def mousePressEvent(self, event):
          self.scene().clear()
  
  image = 'big.png'
  app = QtGui.QApplication([])
  view = ImageView(image)
  view.show()
  app.exec_()
  </code></pre>
  
  <p>Here I load a big (10000x10000px) image and display it in a QGraphicsView, and the scene then can be cleared with a mouse click. If the line <code>f = QtGui.QPixmap(QtGui.QImage(file))</code> is used, then after clearing the scene the memory usage drops from 400 Mb to 20, showing that the image is cleared from memory. However, if I replace that line with <code>f = QtGui.QPixmap(file)</code>, then after the same action memory usage remains at 400 Mb. It doesn't get reallocated later, either - if after clearing the scene I create another ImageView with another big image, another chunk gets allocated. Moreover, when I create QPixmap directly from file, I cannot free up the memory in any way whatsoever - I tried Python's <code>deleteLater</code>, <code>del</code> and gc module. This happens both in PyQt and Pyside. Why is this happening?</p>