add a -jpeg_like option

This option remaps internal parameters to better match
the expected compression curve of JPEG and produce output files
of similar size, but with better quality.

Change-Id: I96a1cbb480b1f6a0c6845a23c33dfd63f197b689

src/webp/encode.h

@@ -121,8 +121,12 @@ struct WebPConfig {
   int partition_limit;    // quality degradation allowed to fit the 512k limit
                           // on prediction modes coding (0: no degradation,
                           // 100: maximum possible degradation).
+  int emulate_jpeg_size;  // If true, compression parameters will be remapped
+                          // to better match the expected output size from
+                          // JPEG compression. Generally, the output size will
+                          // be similar but the degradation will be lower.
 
-  uint32_t pad[8];        // padding for later use
+  uint32_t pad[7];        // padding for later use
 };
 
 // Enumerate some predefined settings for WebPConfig, depending on the type