(trace_move) [GLYPH_DEBUG]: New variable.

(trace_move) [GLYPH_DEBUG]: New variable.
(TRACE_MOVE) [GLYPH_DEBUG]: New macro.
(move_it_in_display_line_to): Record iterator's ascent and descent
before producing glyphs, and restore them when we know the glyph
doesn't fit on the line.
(move_it_to): Restructured so that it's easier to debug.  If
moving to a vpos, and not moving to an x or character position,
stop as soon as the specified vpos is reached; don't move further
into that line because that might change the computed line height.
(try_cursor_movement): New function, extracted from the cursor
movement branch of redisplay_window.  If ending on a partially
visible line, don't try to scroll if the cursor line is taller
than the window.
(redisplay_window): Use try_cursor_movement.