Pairwise optimization should take into account exposure

[Brent Townshend]

When running auto_align_stack or doing any pairwise control-point generation of an HDR sequence, it makes a huge difference to order the pairing by sequential exposures. Autobracket sequences are often stored as a seqence with increasing difference from 0 exposure compensation alternating with under and over exposed frames. If you then optimize in that order you end up matching a heavily under-exposed frame with an heavily-over-exposed one resulting in poor control point generation. By ordering them from least to most exposed, the pairs are the most similar.

[tmodes (tmodes)]

cpfind --multirow does already take exposure values in stacks into account.
Implemented for align_image_stacks in changeset 6944736f760e.