Implement --lossy LZW compression

--lossy option that allows inexact match against LZW dictionary,
which improves compression ratio.
Lossy matching does a bit of 1-dimensional dithering.

This is a very basic implementation that does recursive search of dictionary nodes.

write_compressed_data contains some duplicated code,
because the lossy search function needs to use less optimized code (ignores imageline),
although this probably could be refactored a bit.

The results are pretty good:
— Original: 3.3MB
— Lossy: 1.25MB

Based on gifsicle which implements lossy LZW compression.
It can reduce animgif file sizes by 30%—50% at a cost of some dithering/noise.
— https://pornel.net/lossygif
— https://github.com/pornel/giflossy

Closed: #16

include/lcdfgif/gif.h

@@ -145,6 +145,7 @@ typedef         void (*Gif_ReadErrorHandler)(Gif_Stream* gfs,
 
 typedef struct {
     int flags;
+    int loss;
     void *padding[7];
 } Gif_CompressInfo;
 

src/giffunc.c

@@ -821,6 +821,7 @@ void
 Gif_InitCompressInfo(Gif_CompressInfo *gcinfo)
 {
     gcinfo->flags = 0;
+    gcinfo->loss = 0;
 }
 
 

src/gifsicle.c

@@ -201,6 +201,7 @@ static const char *output_option_types[] = {
 #define RESIZE_TOUCH_OPT        377
 #define RESIZE_TOUCH_WIDTH_OPT  378
 #define RESIZE_TOUCH_HEIGHT_OPT 379
+#define LOSSY_OPT               380
 
 #define LOOP_TYPE               (Clp_ValFirstUser)
 #define DISPOSAL_TYPE           (Clp_ValFirstUser + 1)
@@ -268,6 +269,7 @@ const Clp_Option options[] = {
 
   { "logical-screen", 'S', LOGICAL_SCREEN_OPT, DIMENSIONS_TYPE, Clp_Negate },
   { "loopcount", 'l', 'l', LOOP_TYPE, Clp_Optional | Clp_Negate },
+  { "lossy", 0, LOSSY_OPT, Clp_ValInt, Clp_Optional },
 
   { "merge", 'm', 'm', 0, 0 },
   { "method", 0, COLORMAP_ALGORITHM_OPT, COLORMAP_ALG_TYPE, 0 },
@@ -2073,6 +2075,13 @@ main(int argc, char *argv[])
       }
       break;
 
+    case LOSSY_OPT:
+      if (clp->have_val)
+        gif_write_info.loss = clp->val.i;
+      else
+        gif_write_info.loss = 20;
+      break;
+
       /* RANDOM OPTIONS */
 
      case NO_WARNINGS_OPT: