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

src/dsp/Makefile.am

@@ -0,0 +1,14 @@
+AM_CPPFLAGS = -I$(top_srcdir)/src
+
+libwebpdsp_la_SOURCES = dsp.h cpu.c \
+                        enc.c enc_sse2.c \
+                        dec.c dec_sse2.c \
+                        upsampling.c upsampling_sse2.c \
+                        yuv.h yuv.c
+libwebpdsp_la_LDFLAGS = -version-info 0:0:0 -lm
+libwebpdsp_la_CPPFLAGS = $(USE_EXPERIMENTAL_CODE)
+libwebpdspinclude_HEADERS = ../webp/types.h
+libwebpdspincludedir = $(includedir)/webp
+
+noinst_HEADERS = dsp.h yuv.h
+noinst_LTLIBRARIES = libwebpdsp.la