Avoid adding keyframes on frames with few matchable features.
Avoid adding keyframes on frames with few matchable features.

This change ensures that the average number of matches on the selected
new keyframe is greater than the match requirement.  This avoids
creating keyframes on unmatchable textureless frames.

linus-sherrill/maptk,linus-sherrill/maptk,linus-sherrill/maptk,linus-sherrill/maptk,linus-sherrill/maptk