add Advanced Decoding Interface

You can now use WebPDecBuffer, WebPBitstreamFeatures and WebPDecoderOptions
to have better control over the decoding process (and the speed/quality tradeoff).

WebPDecoderOptions allow to:
 - turn fancy upsampler on/off
 - turn in-loop filter on/off
 - perform on-the-fly cropping
 - perform on the-fly rescale
(and more to come. Not all features are implemented yet).

On-the-fly cropping and scaling allow to save quite some memory
(as the decoding operation will now scale with the output's size, not
the input's one). It saves some CPU too (since for instance,
in-loop filtering is partially turned off where it doesn't matter,
and some YUV->RGB conversion operations are ommitted too).

The scaler uses summed area, so is mainly meant to be used for
downscaling (like: for generating thumbnails or previews).

Incremental decoding works with these new options.
More doc to come soon.

dwebp is now using the new decoding interface, with the new flags:
  -nofancy
  -nofilter
  -crop top left width height
  -scale width height

Change-Id: I08baf2fa291941686f4ef70a9cc2e4137874e85e

README

@@ -144,6 +144,7 @@ options:
   -pass <int> ............ analysis pass number (1..10)
   -partitions <int> ...... number of partitions to use (0..3)
   -crop <x> <y> <w> <h> .. crop picture with the given rectangle
+  -resize <w> <h> ........ resize picture (after any cropping)
   -map <int> ............. print map of extra info.
   -d <file.pgm> .......... dump the compressed output (PGM file).
 
@@ -201,6 +202,25 @@ file test.webp decodes to exactly the same as test_ref.ppm by using:
  ./dwebp test.webp -ppm -o test.ppm
  diff test.ppm test_ref.ppm
 
+The full list of options is available using -h:
+
+> dwebp -h
+Usage: dwebp in_file [options] [-o out_file]
+
+Decodes the WebP image file to PNG format [Default]
+Use following options to convert into alternate image formats:
+  -ppm ......... save the raw RGB samples as color PPM
+  -pgm ......... save the raw YUV samples as a grayscale PGM
+                 file with IMC4 layout.
+ Other options are:
+  -version  .... print version number and exit.
+  -nofancy ..... don't use the fancy YUV420 upscaler.
+  -nofilter .... disable in-loop filtering.
+  -crop <x> <y> <w> <h> ... crop output with the given rectangle
+  -scale <w> <h> .......... scale the output (*after* any cropping)
+  -h     ....... this help message.
+  -v     ....... verbose (e.g. print encoding/decoding times)
+  -noasm ....... disable all assembly optimizations.
 
 Encoding API:
 ===========