Fix VT100Grid scrollRect:downBy: to clear up continuation marks. Modify insertBlankLinesAfterCursor to do nothing if the cursor is outside the scroll region (like xterm), and also to scroll only the region below the cursor. Remove duplicate screenShouldSyncTitle in delegate. Add continuation mark tests to VT100GridTests testScrollRectDownBy.  Add tests for terminalInsertBlankLinesAfterCursor.