Blender Git Loki

Optimisations for building "Long Keyframes"

For a long time, one of the bottlenecks when drawing summary channels in the dopesheet
(especially with many objects) was how the long keyframes feature (i.e showing holds
between keyframes) got built. Specifically, it was the step where we check on the previous
keyframe to see whether there's a hold between those two.

The old code performed some elaborate checks, which made sense back when we used to handle
certain summary channels (e.g. object-action/ipo, and groups IIRC) differently. However,
nowadays, everything just does it by going over the FCurves one by one, so the offending
code wasn't really providing much benefit. Unless I've forgotten some other reason why
that old method is necessary, this commit should provide a decent speedup here, making
things somewhat interactive now (if still a bit jerky).

Other Tweaks:
1) Introduced float-precision threshold when checking to see whether an existing long
keyframe could be reused. This should hopefully reduce the number of fp-jitter issues
when creating summaries for many channels, reducing the number of duplicates created.
2) Precompute colours used for shading the long keyframes, instead of recomputing for
each block.