calling QImageReader::supportedImageFormats() before creating a QApplication caches an incomplete list of supported formats

If you call QImageReader::supportedImageFormats() before creating an image, you get only a list of formats that are supported natively, without plugins, AND QImageReader remembers this list and does not support images supported by plugins even after you create a QApplication().

(in Pythonesque syntax)

formats1 = QImageReader.supportedImageFormats()
app = QApplication([])
formats2 = QImageReader.supportedImageFormats()
p = QPixmap('something.jpg') # won't work

In the above code, formats1 and formats2 will be the same incomplete list, without JPEG. If you comment out the "formats1" line, formats2 will include JPEG.

The easiest fix is to document that QImageReader::supportedImageFormats() should not be called before QApplication() is called.

This was previously mentioned years ago here: http://lists.trolltech.com/qt-interest/2005-11/thread00422-0.html, though no bug was filed as far as I can tell. I encountered this independently.