QWindowsPrintDevice::printableMargins() leaks GDI objects (HDC)

QWindowsPrintDevice::printableMargins allocates a HDC with CreateDC, and then incorrectly tries to release it with ReleaseDC instead of DeleteDC. This results in a GDI object leak every time the function is called, eventually leading to a crash of the application.

See <http://msdn.microsoft.com/en-us/library/windows/desktop/dd162920(v=vs.85).aspx> :

An application cannot use the ReleaseDC function to release a DC that was created by calling the CreateDC function; instead, it must use the DeleteDC function.

The attached patch against Qt 5.3 replaces ReleaseDC with DeleteDC, which fixes the leak we observed.