Keyframe selection improvements

Added additional criteria, which ignores
keyframe pair if success intersection factor
is lower than current candidate pair factor.

This solves issue with keyframe pair at which
most of the tracks are intersecting behind the
camera is accepted (because variance in this
case is really small),

Also tweaked generalized inverse function,
which now doesn't scale epsilon by maximal
matrix element. This gave issues at really bad
candidates with unstable covariance. In this
case almost all eigen values getting zeroed
on inverse leading to small expected error.