Revert to signed span positioning.

In the context of FT_Outline_Render, the outline can be placed
arbitrarily and the resulting spans should be signed. This partially
reverts ef04e4eb2056 and fixes #1410.  However, for internal use,
when outlines are placed in the first quadrant, we still can interpret
positions as unsigned.

* include/freetype/ftimage.h (FT_Span): Use signed position again.
* src/smooth/ftgrays.c (gray_sweep_direct): Sync with FT_Span.
* src/smooth/ftsmooth.c (ft_smooth_lcd_spans, ft_smooth_lcdv_spans,
ft_smooth_overlap_spans): Prevent sign extension.

include/freetype/ftimage.h

diff --git a/include/freetype/ftimage.h b/include/freetype/ftimage.h
index 1fff76b..7c41912 100644
--- a/include/freetype/ftimage.h
+++ b/include/freetype/ftimage.h
@@ -875,7 +875,7 @@
    */
   typedef struct  FT_Span_
   {
-    unsigned short  x;
+    short           x;
     unsigned short  len;
     unsigned char   coverage;
 

src/smooth/ftgrays.c

diff --git a/src/smooth/ftgrays.c b/src/smooth/ftgrays.c
index b77687f..53948d2 100644
--- a/src/smooth/ftgrays.c
+++ b/src/smooth/ftgrays.c
@@ -1805,7 +1805,7 @@
           FT_FILL_RULE( coverage, cover, fill );
 
           span[n].coverage = (unsigned char)coverage;
-          span[n].x        = (unsigned short)x;
+          span[n].x        = (short)x;
           span[n].len      = (unsigned short)( cell->x - x );
 
           if ( ++n == FT_MAX_GRAY_SPANS )
@@ -1824,7 +1824,7 @@
           FT_FILL_RULE( coverage, area, fill );
 
           span[n].coverage = (unsigned char)coverage;
-          span[n].x        = (unsigned short)cell->x;
+          span[n].x        = (short)cell->x;
           span[n].len      = 1;
 
           if ( ++n == FT_MAX_GRAY_SPANS )

src/smooth/ftsmooth.c

diff --git a/src/smooth/ftsmooth.c b/src/smooth/ftsmooth.c
index 25c1e5e..f5a3de6 100644
--- a/src/smooth/ftsmooth.c
+++ b/src/smooth/ftsmooth.c
@@ -124,7 +124,8 @@
 
 
     for ( ; count--; spans++ )
-      for ( dst = dst_line + spans->x * 3, w = spans->len; w--; dst += 3 )
+      for ( dst = dst_line + (unsigned short)spans->x * 3,
+            w = spans->len; w--; dst += 3 )
         *dst = spans->coverage;
   }
 
@@ -290,7 +291,8 @@
 
 
     for ( ; count--; spans++ )
-      for ( dst = dst_line + spans->x, w = spans->len; w--; dst++ )
+      for ( dst = dst_line + (unsigned short)spans->x,
+            w = spans->len; w--; dst++ )
       {
         dst[0] += ( spans->coverage * target->wght[0] + 85 ) >> 8;
         dst[1] += ( spans->coverage * target->wght[1] + 85 ) >> 8;
@@ -385,7 +387,8 @@
 
 
     for ( ; count--; spans++ )
-      for ( dst = dst_line + spans->x, w = spans->len; w--; dst++ )
+      for ( dst = dst_line + (unsigned short)spans->x,
+            w = spans->len; w--; dst++ )
       {
         dst[        0] += ( spans->coverage * target->wght[0] + 85 ) >> 8;
         dst[    pitch] += ( spans->coverage * target->wght[1] + 85 ) >> 8;
@@ -473,7 +476,7 @@
 
 
     unsigned char*  dst = target->origin - ( y / SCALE ) * target->pitch;
-    unsigned short  x;
+    unsigned int    x, i;
     unsigned int    cover, sum;
 
 
@@ -487,8 +490,9 @@
       cover = ( spans->coverage + SCALE * SCALE / 2 ) / ( SCALE * SCALE );
       for ( x = 0; x < spans->len; x++ )
       {
-        sum                           = dst[( spans->x + x ) / SCALE] + cover;
-        dst[( spans->x + x ) / SCALE] = (unsigned char)( sum - ( sum >> 8 ) );
+        i      = ( (unsigned short)spans->x + x ) / SCALE;
+        sum    = dst[i] + cover;
+        dst[i] = (unsigned char)( sum - ( sum >> 8 ) );
       }
     }
   }