stereo: Produce lowres is no longer used in multiple places

Thus produce lowres should no longer be inside stereo.{cc,h}.
gitignore: Update to represent current state of affairs.
Common.h: Block write image with georef and nodata
dg_mosaic.py.in: Compensate for pitch diff
 - added an --output-prefix option
- check that DETORIGINX, PD, EPH, and ATT are same among xml files