stereo_corr: Consolidate with stereo_corr_sub

The executable stereo_corr and stereo_corr_sub were sharing a good chunk of code. Where they diferred, stereo_corr_sub was not doing the right thing, it was computing D_sub even if the user explicitely asked not to.

I integrated them into one single function.

Long term, stereo_corr_sub may not be needed at all, one could just call stereo_corr with a flag saying --compute-lowres-disparity-only