Client-side handle to a drawable window with a backup bitmap.

A backed-up window is associated with a backup bitmap which retains the contents of the window. When an area of the window is invalidated, the window server uses the bitmap directly to update the window without requiring an application redraw. The backup bitmap is owned by the font and bitmap server and resides on the font and bitmap server's shared heap, which is accessible to all client programs: functions are provided to allow the client application to access this bitmap directly and perform updates to the window under application control.