add intrinsics version of SimpleHFilter16NEON()

It's disable for now, because it crashes gcc-4.6.3 during compilation
with -O2 or -O3. It's been tested OK with -O1.

Code is still globally disabled with USE_INTRINSICS, though.

Change-Id: I3ca6cf83f3b9545ad8909556f700758b3cefa61c
windows: fix dll builds

WebPSafe* need to be marked external to allow mux/demux to access them
through libwebp.dll

Change-Id: Ib6620e00d376f7aa5a0550e1e244f759977f97a0
Merge "add intrinsics version of SimpleHFilter16NEON()"
dec_neon: add strong loopfilter intrinsics

vertical only currently, 2.5-3% faster
placed under USE_INTRINSICS as this change depends on the simple
loopfilter
improves the simple loopfilter slightly thanks to some reorganization

Change-Id: I6611441fa54228549b21ea74c013cb78d53c7155