prepare experimentation with yuv444 / 422

+ add a simple rescaling function: WebPPictureRescale() for encoding
+ clean-up the memory managment around the alpha plane
+ fix some includes path by using "../webp/xxx.h" instead of "webp/xxx.h"

New flags for 'cwebp':
 -resize <width> <height>
 -444  (no effect)
 -422  (no effect)
 -400

Change-Id: I25a95f901493f939c2dd789e658493b83bd1abfa

Android.mk

@@ -2,15 +2,18 @@ LOCAL_PATH:= $(call my-dir)
 
 include $(CLEAR_VARS)
 LOCAL_SRC_FILES := \
+	src/dec/alpha.c \
 	src/dec/bits.c \
 	src/dec/dsp.c \
 	src/dec/frame.c \
 	src/dec/idec.c \
+	src/dec/layer.c \
 	src/dec/quant.c \
 	src/dec/tree.c \
 	src/dec/vp8.c \
 	src/dec/webp.c \
 	src/dec/yuv.c \
+	src/enc/alpha.c \
 	src/enc/analysis.c \
 	src/enc/bit_writer.c \
 	src/enc/config.c \
@@ -18,6 +21,7 @@ LOCAL_SRC_FILES := \
 	src/enc/filter.c \
 	src/enc/frame.c \
 	src/enc/iterator.c \
+	src/enc/layer.c \
 	src/enc/picture.c \
 	src/enc/quant.c \
 	src/enc/syntax.c \