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/config.c

@@ -10,7 +10,7 @@
 // Author: Skal (pascal.massimino@gmail.com)
 
 #include <assert.h>
-#include "webp/encode.h"
+#include "../webp/encode.h"
 
 #if defined(__cplusplus) || defined(c_plusplus)
 extern "C" {