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

Makefile.vc

@@ -170,6 +170,7 @@ DEMUX_OBJS = \
     $(DIROBJ)\demux\demux.obj \
 
 DSP_DEC_OBJS = \
+    $(DIROBJ)\dsp\alpha_processing.obj \
     $(DIROBJ)\dsp\cpu.obj \
     $(DIROBJ)\dsp\dec.obj \
     $(DIROBJ)\dsp\dec_clip_tables.obj \
@@ -228,7 +229,6 @@ MUX_OBJS = \
     $(DIROBJ)\mux\muxread.obj \
 
 UTILS_DEC_OBJS = \
-    $(DIROBJ)\utils\alpha_processing.obj \
     $(DIROBJ)\utils\bit_reader.obj \
     $(DIROBJ)\utils\color_cache.obj \
     $(DIROBJ)\utils\filters.obj \