add a -z option to cwebp, and WebPConfigLosslessPreset() function

These are presets for lossless coding, similar to zlib.
The shortcut for lossless coding is now, e.g.:
   cwebp -z 5 in.png -o out_lossless.webp

There are 10 possible values for -z parameter:
   0 (fastest, lowest compression)
to 9 (slowest, best compression)

A reasonable tradeoff is -z 6, e.g.
-z 9 can be quite slow, so use with care.

This -z option is just a shortcut for some pre-defined
'-lossless -m xx -q yy' combinations.

Change-Id: I6ae716456456aea065469c916c2d5ca4d6c6cf04

src/webp/encode.h

@@ -20,7 +20,7 @@
 extern "C" {
 #endif
 
-#define WEBP_ENCODER_ABI_VERSION 0x0202    // MAJOR(8b) + MINOR(8b)
+#define WEBP_ENCODER_ABI_VERSION 0x0203    // MAJOR(8b) + MINOR(8b)
 
 // Note: forward declaring enumerations is not allowed in (strict) C and C++,
 // the types are left here for reference.
@@ -167,6 +167,14 @@ static WEBP_INLINE int WebPConfigPreset(WebPConfig* config,
                                 WEBP_ENCODER_ABI_VERSION);
 }
 
+// Activate the lossless compression mode with the desired efficiency level
+// between 0 (fastest, lowest compression) and 9 (slower, best compression).
+// A good default level is '6', providing a fair tradeoff between compression
+// speed and final compressed size.
+// This function will overwrite several fields from config: 'method', 'quality'
+// and 'lossless'. Returns false in case of parameter error.
+WEBP_EXTERN(int) WebPConfigLosslessPreset(WebPConfig* config, int level);
+
 // Returns true if 'config' is non-NULL and all configuration parameters are
 // within their valid ranges.
 WEBP_EXTERN(int) WebPValidateConfig(const WebPConfig* config);