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/Makefile.am

@@ -1,11 +1,12 @@
-SUBDIRS = dec enc
+SUBDIRS = dec enc dsp
 
 AM_CPPFLAGS = -I$(top_srcdir)/src
 lib_LTLIBRARIES = libwebp.la
 
 libwebp_la_SOURCES =
 libwebp_la_LIBADD = dec/libwebpdecode.la \
-                    enc/libwebpencode.la
+                    enc/libwebpencode.la \
+                    dsp/libwebpdsp.la
 libwebp_la_LDFLAGS = -version-info 0:0:0
 libwebpinclude_HEADERS = webp/types.h webp/decode.h webp/decode_vp8.h \
                          webp/encode.h