convert canvas items that compose the playhead (EditorCursor, more generally) to pointers, since canvas items are owned by the group they belong to. This avoids a double-free during deletion
make measure line positioning use Editor::sample_to_pixel() and avoid extra 0.5 pixel shift
correct delete/free mess
fix up major thinko's in ArdourCanvas::Group's handling of deletion (both its own, and child items)
slight code tidy for Line::render() ... don't use 0.5 pixel adjustment if the line width > 1.0