Fix for 687368 (large rendering of platfonts)

Reviewers,

   As discussed last week, here is a simple patch which simply
disables xfonts (platfonts) when antialiasing is in effect. This fixes
bug 687368, and also avoids quality degradation arising from mixing
platfonts and antialiased fonts.

   The scale problem was introduced when the antialiasing logic was
changed; previously, the "pair" at the time of xfont lookup
represented the metrics at the device resolution, but now the "pair"
is computed based on the metrics at the supersampling resolution.
I considered diving in and making the pair for xfont lookup reflect
the device resolution, but the logic looked fairly delicate, and I
didn't want to break anything.

Raph

Index: src/gxchar.c
===================================================================
RCS file: /cvs/ghostscript/gs/src/gxchar.c,v
retrieving revision 1.40
diff -C2 -r1.40 gxchar.c
*** a/src/gxchar.c	13 Mar 2004 18:28:52 -0000	1.40
--- b/src/gxchar.c	1 Apr 2004 16:22:24 -0000
***************
*** 873,876 ****
--- 873,877 ----
      int code;
      cached_char *cc;
+     gs_log2_scale_point log2_scale;
  
      if (penum->charpath_flag == cpm_show && SHOW_USES_OUTLINE(penum)) {
***************
*** 927,931 ****
  		    {
  			int alpha_bits, depth;
- 			gs_log2_scale_point log2_scale;
  			gs_fixed_point subpix_origin;
  
--- 928,931 ----
***************
*** 948,951 ****
--- 948,955 ----
  			/* If possible, try for an xfont before */
  			/* rendering from the outline. */
+ 
+ 		        /* If antialiasing is in effect, don't use xfont */
+ 		        if (log2_scale.x + log2_scale.y > 0)
+ 			    goto no_cache;
  			if (pfont->ExactSize == fbit_use_outlines ||
  			    pfont->PaintType == 2