lossy decoding: correct alpha-rescaling for YUVA format

The luminance needs to be pre- and post- multiplied by
the alpha value in case of rescaling, for proper averaging.

Also:
- removed util/alpha_processing and moved it to dsp/
- removed WebPInitPremultiply() which was mostly useless
and merged it with the new function WebPInitAlphaProcessing()

Change-Id: If089cefd4ec53f6880a791c476fb1c7f7c5a8e60

Android.mk

@@ -32,6 +32,7 @@ LOCAL_SRC_FILES := \
     src/dec/vp8.c \
     src/dec/vp8l.c \
     src/dec/webp.c \
+    src/dsp/alpha_processing.c \
     src/dsp/cpu.c \
     src/dsp/dec.c \
     src/dsp/dec_clip_tables.c \
@@ -68,7 +69,6 @@ LOCAL_SRC_FILES := \
     src/enc/tree.c \
     src/enc/vp8l.c \
     src/enc/webpenc.c \
-    src/utils/alpha_processing.c \
     src/utils/bit_reader.c \
     src/utils/bit_writer.c \
     src/utils/color_cache.c \