NEON: fix overflow in SSE NxN calculation
NEON: fix overflow in SSE NxN calculation

vmlal_u8() is prone to overflow during the accumulation.
There was a mismatch happening at low q mostly. Because in this
case the distortion is important and the accumulated sum was
later than 16bit-unsigned.

Change-Id: I1a08a2f744bcdf0b26647e61b9ee92a0c2e28fe8

PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp