Thus it now actually takes a buffer pointer also on iOS, like on Linux
and Android. Less confusing, more uniform. Add a separate iOS-specific
paintTileToCGContext() method to LibreOfficeKitDocumentClass that
takes a CGContextRef. Adapt callers correspondingly. (The
LibreOfficeLight code in particular needs the paintTileToCGContext().)

Change-Id: I81084806d37b9aac9f2b2bc03d0c262e991eec81