(x_append_glyph, x_append_composite_glyph)

(x_append_glyph, x_append_composite_glyph)
(x_produce_image_glyph, x_append_stretch_glyph): Accomodate to
changes in struct glyph starting 1999-12-27.  Some bit-fields of
struct glyph were not set, which made glyphs unequal when compared
with GLYPH_EQUAL_P.  Redisplay outputs such glyphs, and flickering
effects were the result.  This also depended on the contents of
memory returned by xmalloc.  If flickering happens again, activate
the code in clear_glyph_row that's in #if 0.  If the flickering is
gone with that, chances are that it is caused by something
similar.