The row, E, F block pattern 2, 4, 6 is a common one for
bundle adjustment with reprojection error (2 residuals),
homogeneous 3d points (4 params in the E-block), and camera
poses (3 rotation + 3 position = 6 params for the
F-block). This provides a major speedup for BA in the
TheiaSfM library and likely in other applications.
Change-Id: If5df8bfadc7f154856b74c3b38479c14856db47d
Chris Sweeney