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

src/dec/Makefile.am

@@ -1,7 +1,8 @@
 AM_CPPFLAGS = -I$(top_srcdir)/src
 
 libwebpdecode_la_SOURCES = bits.h vp8i.h yuv.h bits.c dsp.c frame.c \
-                          quant.c tree.c vp8.c webp.c yuv.c idec.c alpha.c
+                          quant.c tree.c vp8.c webp.c yuv.c idec.c alpha.c \
+                          layer.c
 libwebpdecode_la_LDFLAGS = -version-info 0:0:0
 libwebpdecode_la_CPPFLAGS = $(USE_EXPERIMENTAL_CODE)
 libwebpdecodeinclude_HEADERS = ../webp/decode.h ../webp/decode_vp8.h ../webp/types.h