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/enc/Makefile.am

@@ -3,7 +3,7 @@ AM_CPPFLAGS = -I$(top_srcdir)/src
 libwebpencode_la_SOURCES = analysis.c bit_writer.c bit_writer.h \
                           config.c cost.c cost.h dsp.c dsp_sse2.c filter.c \
                           frame.c iterator.c picture.c quant.c  \
-                          syntax.c tree.c vp8enci.h webpenc.c alpha.c
+                          syntax.c tree.c vp8enci.h webpenc.c alpha.c layer.c
 libwebpencode_la_LDFLAGS = -version-info 0:0:0 -lm
 libwebpencode_la_CPPFLAGS = $(USE_EXPERIMENTAL_CODE)
 libwebpencodeinclude_HEADERS = ../webp/encode.h ../webp/types.h