Backup behind windows

that when a dialogue that is fade behind disappears no redraws should be issued to the rest of the displayed windows in order for themselves to be redrawn unfaded.

Backup behind windows take a copy of the screen they are going to obliterate and blit it back to the screen when they disappear rather than issuing a redraw to the application behind. They may backup the part of the screen obscured by the window or the whole of screen. Backing up the whole screen is useful for windows that require the rest of the screen to fade when they are displayed. such as fading the window behind the dialog with focus.

The function RWindowBase::EnableBackup() sets a window to have one of at least two different types of backup, EWindowBackupAreaBehind which is just the area behind this window (the default), and EWindowBackupFullScreen, which is the whole screen.

EWindowBackupFullScreen allows yot to have a fade behind window and when it becomes unfaded behind, the screen will be redrawn by the window server instead of issuing redraw events to all the apps. The window fade behind backup flag should be set for a window before the window is activated and before calling the fade behind RWindowBase::Fadebehind() function. If it is called after activating the window then the backup will not include the part of the screen obscured by the window itself.