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

man/cwebp.1

@@ -1,5 +1,5 @@
 .\"                                      Hey, EMACS: -*- nroff -*-
-.TH CWEBP 1 "March  28, 2011"
+.TH CWEBP 1 "June  20, 2011"
 .SH NAME
 cwebp \- compress an image file to a WebP file
 .SH SYNOPSIS
@@ -102,8 +102,8 @@ options \fB\-size\fP or \fB\-psnr\fP. Maximum value is 10.
 .TP
 .B \-crop x_position y_position width height
 Crop the source to a rectangle with top-left corner at coordinates
-(x_position, y_position) and size width x height. This cropping area must
-be fully contained within the source rectangle.
+(\fBx_position\fP, \fBy_position\fP) and size \fBwidth\fP x \fBheight\fP.
+This cropping area must be fully contained within the source rectangle.
 .TP
 .B \-s width height
 Specify that the input file actually consists of raw Y'CbCr samples following