sparse_disp: minor changes + better handling of failed matches

Gaps where the epipolar test failed are now marked in D_sub_spread with zeros.
Other improvements:
-- only 25% of the search window must be marked valid for a search to occur (had been 50%)
-- the valid mask is diated before writing or before being used to mask D_sub_spread
-- initial search pts now can be as little as coarse_skip/4 from the image edges
-- A larger percentile fraction of disparites are used in calculating the epipolar vector