bit_reader.h: cosmetics

- use const where applicable
- drop unnecessary string.h include

Change-Id: I560eef84fe17d3925768f6817c02ea79604c4379
Support for "Do not blend" in mux and demux libraries

Change-Id: I9566a8e2d059fe1ebd9ca99c7e13311bf3f8f281
Pack code & extra_bits to Struct (VP8LPrefixCode).

Also created variant VP8LPrefixEncodeBits that returns the
code & extra_bits only.
There's no impact on compression density and compression speed.

Change-Id: I2cafdd3438ac9270cd72ad9d57b383cdddfdfa4c
Merge "Support for "Do not blend" in mux and demux libraries"
Support for 'do not blend' option in vwebp

Change-Id: I563e192fa727816d11d8647c559ec407205ef40b
Merge "Support for 'do not blend' option in vwebp"
fix const-ness

Change-Id: I09c85fdb90b4380c8fc03c8b5652bb9cf29b2021
cosmetics

Change-Id: Ib6fa2a7e1db8c5c48607c7097520ffca34a6cb66
Merge "bit_reader.h: cosmetics"
Merge "cosmetics"
Merge branch 'master' of http://github.com/gitmacx/libwebp