Blender Git Loki

Fix T39207: FCurve evaluation regressions following 2aff243

This commit attempts to fix some of the FCurve evaluation regressions arising from
an earlier commit to speed up the process using binary search. Further tweaks may still
be needed though to get this to an acceptable level of reliability (namely, tuning the
threshold defining which keyframes get considered "close together"). Since we're still
in an early stage of the 2.71 dev cycle, for now it's still worth trying to get this
working instead of simply reverting this (which can still be done later if it proves too
problematic).

Specific fixes:
* The previous code was somewhat dangerous in that it allowed out-of-bounds accessing
of memory when a == 0. It turns out this was more common than originally anticipated
(as the assert I added here ended up failing in the "action_bug.blend" file in the report)
* Tweaked the code used to test for closely-spaced points so that the "Clive.blend" example
for driver curves won't fail. The approach used here has the downside though that
since "exact" uses a might coarser threshold for equality, there may be some precision
loss issues causing backwards compat issues (namely with closely spaced keyframes, or
for certain NLA strips).

For now, I've left in some debug prints that can be enabled by running Blender in debug
mode (i.e. "blender -d"), which can provide some useful tuning info should we need to
look more into our approach here.