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

Makefile.vc

@@ -121,6 +121,8 @@ X_OBJS= \
 	$(DIROBJ)\dec\webp.obj \
 	$(DIROBJ)\dec\yuv.obj \
 	$(DIROBJ)\dec\idec.obj \
+	$(DIROBJ)\dec\alpha.obj \
+	$(DIROBJ)\dec\layer.obj \
 	$(DIROBJ)\enc\analysis.obj \
 	$(DIROBJ)\enc\bit_writer.obj \
 	$(DIROBJ)\enc\config.obj \
@@ -134,6 +136,8 @@ X_OBJS= \
 	$(DIROBJ)\enc\syntax.obj \
 	$(DIROBJ)\enc\tree.obj \
 	$(DIROBJ)\enc\webpenc.obj \
+	$(DIROBJ)\enc\alpha.obj \
+	$(DIROBJ)\enc\layer.obj \
 	$(RESOURCE)
 
 EXAMPLES_OBJS = \