Using QWhatsThis on certain X11 setups may result in X11 errors

Using QWhatsThis on certain X11 setups as explained below may result in X11 errors such as

Major opcode of failed request: 62 (X_CopyArea)
Serial number of failed request: 2534
Current serial number in output stream: 2536

This problem may happen when the root window has a depth of 24bit while the application uses a visual with a depth of 8bit. QWhatsThis will call QPixmap::grabWindow() on the root window, while the default depth of all QPixmaps will be the same as the application itself uses. This means that Qt ends up attempting to grab data from a window using a 24bit visual into a 8bit QPixmap.