create a separate libwebpdsp under src/dsp

Gathers all DSP-related function (and SSE2 implementations).
Clean-up some unwanted symbolic dependencies so that webp_encode,
webp_decode and webp_dsp are truly independent libraries.

+ opportunistic clean-up:
  * remove unneeded VP8DspInitTables(), now integrated in VP8DspInit()
  * make consistent use of VP8GetCPUInfo() in the various DspInit() funcs
  * change OUT macro to DST

Android.mk

@@ -14,8 +14,12 @@ LOCAL_SRC_FILES := \
 	src/dec/webp.c \
 	src/dec/io.c \
 	src/dec/buffer.c \
-	src/dec/yuv.c \
 	src/dec/thread.c \
+	src/dsp/yuv.c \
+	src/dsp/upsampling.c \
+	src/dsp/cpu.c \
+	src/dsp/dec.c \
+	src/dsp/enc.c \
 	src/enc/alpha.c \
 	src/enc/analysis.c \
 	src/enc/bit_writer.c \