add light filtering NEON intrinsics

disabled for now (but tested OK), thanks to the USE_INTRINSICS #define
We'll activate the code when we're on par with non-intrinsics

Change-Id: Idbfb9cb01f4c7c9f5131b270f8c11b70d0d485ff
add some colorspace conversion functions in NEON

new file: lossless_neon.c
speedup is ~5%

gcc 4.6.3 seems to be doing some sub-optimal things here,
storing register on stack using 'vstmia' and such.
Looks similar to gcc.gnu.org/bugzilla/show_bug.cgi?id=51509

I've tried adding  -fno-split-wide-types and it does help
the generated assembly. But the overall speed gets worse with
this flag. We should only compile lossless_neon.c with it -> urk.

Change-Id: I2ccc0929f5ef9dfb0105960e65c0b79b5f18d3b0
SSE2 variants of Subtract-Green: Rectify loop condition

When 4 pixels are left, they should be processed with SSE2.

Decoding is marginally faster (~0.4%).
Encoding speed: No observable difference.

Change-Id: I3cf21c07145a560ff795451e65e64faf148d5c3e
Merge "add some colorspace conversion functions in NEON"