When in TweakMode on NLA strips that had an offset, it was not possible to select
those keyframes in the Summary Channel in the Dope Sheet.
The main gist of it is that the current code is from before the summary track was
introduced, and so could assume that ANIM_nla_mapping_get() would work for all channels
present. Thus, simply converting the clicked frame to nla-mapped time once would be
enough. However, for summary channels, nla-mapping_get() doesn't do anything, since
we can potentially include keyframes from several different objects!
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