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/frame.c

@@ -568,6 +568,14 @@ int VP8EncLoop(VP8Encoder* const enc) {
     } else {   // reset predictors after a skip
       ResetAfterSkip(&it);
     }
+#ifdef WEBP_EXPERIMENTAL_FEATURES
+    if (enc->has_alpha_) {
+      VP8EncCodeAlphaBlock(&it);
+    }
+    if (enc->use_layer_) {
+      VP8EncCodeLayerBlock(&it);
+    }
+#endif
     StoreSideInfo(&it);
     VP8StoreFilterStats(&it);
     VP8IteratorExport(&it);