(start_display): WHen starting display on a continuation

(start_display): WHen starting display on a continuation
line, clear ascent and descent members of the iterator structure;
the height of the continued line does not affect the height of the
continuation line.
(make_cursor_line_fully_visible): Do nothing if cursor is on a
line taller than the window.
(redisplay_window) <forced window start>: Handle case that the
middle of the window is not found in any row.
(dump_glyph_row): Show more information.
(compute_line_metrics): Use MATRIX_FIRST_TEXT_ROW to determine the
first text line in a glyph matrix.