EXPERIMENTAL: add support for alpha channel

This is a (minor) bitstream change: if the 'color_space' bit is set to '1'
(which is normally an undefined/invalid behaviour), we add extra data at the
end of partition #0 (so-called 'extensions')

Namely, we add the size of the extension data as 3 bytes (little-endian),
followed by a set of bits telling which extensions we're incorporating.
The data then _preceeds_ this trailing tags.

This is all experimental, and you'll need to have
'#define WEBP_EXPERIMENTAL_FEATURES' in webp/types.h to enable this code
(at your own risk! :))

Still, this hack produces almost-valid WebP file for decoders that don't
check this color_space bit. In particular, previous 'dwebp' (and for instance
Chrome) will recognize this files and decode them, but without the alpha
of course. Other decoder will just see random extra stuff at the end of
partition #0.

To experiment with the alpha-channel, you need to compile on Unix platform
and use PNGs for input/output.

If 'alpha.png' is a source with alpha channel, then you can try (on Unix):

  cwebp alpha.png -o alpha.webp
  dwebp alpha.webp -o test.png

cwebp now has a '-noalpha' flag to ignore any alpha information from the
source, if present.

More hacking and experimenting welcome!

Change-Id: I3c7b1fd8411c9e7a9f77690e898479ad85c52f3e

README

@@ -112,6 +112,8 @@ options:
   -f <int> ............... filter strength (0=off..100)
   -sharpness <int> ....... filter sharpness (0:most .. 7:least sharp)
   -strong ................ use strong filter instead of simple.
+  -alpha_comp <int> ...... set the transparency-compression
+  -noalpha ............... discard any transparency information.
   -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
@@ -120,6 +122,8 @@ options:
 
   -short ................. condense printed message
   -quiet ................. don't print anything.
+  -version ............... print version number and exit.
+  -noasm ................. disable all assembly optimizations.
   -v ..................... verbose, e.g. print encoding/decoding times
 
 Experimental Options: