Render Stage Screen Device abstraction.

Render Stages provide this interface as an abstraction of the target screen. The screen may not be pixel based, in which case read back of pixel data may not be a reflection of what appears on the screen. The screen might also only be partially readable, for example external video surfaces may be blanked out when a read back is done. The intention of the CopyScreenToBitmap methods is to provide a means to obtain a snapshot of the entire screen for architectures where a composition of the screen can be done once followed by a read back of the entire screen, as opposed to servicing a sequence of GetScanLine calls which may be prohibitively expensive.