function CreateROIBitmap(Rect: TRect; ROIPixelFormat: TIEPixelFormat; PerformAlphaCompositing: boolean = false): TIEBitmap;
Creates a TIEBitmap object containing a copy of the specified area.
When the returned object is destroyed the content is copied back to the original bitmap.
If PerformAlphaCompositing is True then the resulting bitmap is black and fully transparent. An alpha compose will be performed when bitmap is released.
ROIPixelFormat is ignored when PerformAlphaCompositing is True (it is always ie32RGB).