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/webpi.h

@@ -16,7 +16,7 @@
 extern "C" {
 #endif
 
-#include "webp/decode_vp8.h"
+#include "../webp/decode_vp8.h"
 
 // Decoding output parameters.
 typedef struct {