The convolve filters generated by loop_wiener_filter_tile
are not compatible with some existing convolve implementations
(they can have coefficients >128, sums of (certain subsets of)
coefficients >128, etc.)

So we implement a new variant, which takes a filter with 128
subtracted from its central element and which adds an extra copy
of the source just before clipping to a pixel (reinstating the
128 we subtracted). This should be easy to adapt from the existing
convolve functions, and this patch includes SSE2 highbd and
SSSE3 lowbd implementations.

Change-Id: I0abf4c2915f0665c49d88fe450dbc77b783f69e1