reducing equality tests in displaying text

[YAMAMOTO Mitsuharu <mituharu@math.s.chiba-u.ac.jp>]

I experimented with some performance tool and noticed that there were
frequent equality tests in displaying text.  I think most of them can
safely be reduced.

1. Each call of assoc_no_quit (charset,_encoding_charset_alist) in
   fontset.c can be replaced with assq_no_quit because
   Vfont_encoding_charset_alist is a symbol-keyed alist.

2. Also in fontset.c:

   472	  if (c >= 0)
   473	    font_group = char_table_ref_and_range (base_fontset, c, &from, &to);
   474	  else
   475	    font_group = FONTSET_FALLBACK (base_fontset);
   476	  if (NILP (font_group))
   477	    return Qnil;

   If `font_group' becomes Qnil in line 473, `from' and `to' are not
   used at all.  So the most of the task of char_table_ref_and_range
   is wasted for this case.  Because font_group == Qnil case happens
   frequently, the char_table_ref_and_range call should be avoided for
   this case.

3. Despite its argument name, `has_char' functions in font backend
   drivers are mostly called for a font object instead of a font
   entity.  Some font backend drivers could make use of this fact:
   e.g., ftfont_has_char can avoid frequent assoc_no_quit calls if
   struct ft_font has a reference to fc_charset in ft_face_cache.

BTW, I suspect Ffontset_font is broken.  I couldn't get any meaningful
result from (fontset-font t ?a), for example.

				     YAMAMOTO Mitsuharu
				mituharu@math.s.chiba-u.ac.jp