Better preserve position when making an image empty.

Previous code could set left to (uint16_t) -1 on a total crop.
Don't!

src/giffunc.c

@@ -466,8 +466,10 @@ Gif_CopyImage(Gif_Image *src)
 void Gif_MakeImageEmpty(Gif_Image* gfi) {
     Gif_ReleaseUncompressedImage(gfi);
     Gif_ReleaseCompressedImage(gfi);
-    gfi->left = gfi->top = 0;
-    gfi->width = gfi->height = 1;
+    gfi->left = gfi->left < 0xFFFE ? gfi->left : 0xFFFE;
+    gfi->top = gfi->top < 0xFFFE ? gfi->top : 0xFFFE;
+    gfi->width = 1;
+    gfi->height = 1;
     gfi->transparent = 0;
     Gif_CreateUncompressedImage(gfi, 0);
     gfi->img[0][0] = 0;

src/support.c

@@ -1421,9 +1421,9 @@ analyze_crop(int nmerger, Gt_Crop* crop, int compress_immediately)
         }
       }
 
-    if (t > b)
+    if (t > b) {
       crop->w = crop->h = 0;
-    else {
+    } else {
       crop->x = l;
       crop->y = t;
       crop->w = r - l;

src/xform.c

@@ -262,18 +262,18 @@ crop_image(Gif_Image* gfi, Gt_Frame* fr, int preserve_total_crop)
             gfi->img[j] = old_img[c.y + j] + c.x;
         gfi->img[c.h] = 0;
         Gif_DeleteArray(old_img);
+        gfi->left += c.x - fr->left_offset;
+        gfi->top += c.y - fr->top_offset;
         gfi->width = c.w;
         gfi->height = c.h;
-    } else if (preserve_total_crop)
+    } else if (preserve_total_crop) {
         Gif_MakeImageEmpty(gfi);
-    else {
+    } else {
         Gif_DeleteArray(gfi->img);
         gfi->img = 0;
         gfi->width = gfi->height = 0;
     }
 
-    gfi->left += c.x - fr->left_offset;
-    gfi->top += c.y - fr->top_offset;
     return gfi->img != 0;
 }