Add new verticalalignment option 'cap' for OpenGL plotting. * libinterp/interp-core/txt-eng-ft.cc(ft_render::text_to_pixels): Implement new 'cap' vertical alignment which is glyph_max_height - glyph_descender_height. This approximates height of a capital letter. * libinterp/interpfcn/graphics.cc (text::properties::update_text_extent): Add 'cap' to if/else if/ tree checking possible verticalalignment options.