Animation: add function for deduplicating FCurve keys

Introduce `BKE_fcurve_deduplicate_keys()` to merge keys that are on the
same time, or within the time comparison threshold (1/100th of a frame).

When merging two consecutive keys, the last one 'wins' and determines
the final key *value*. The first key's *time* is retained, to ensure the
reference point for the next comparisons is stable. The only exception
here is when there is a key exactly on an integer frame number, in which
case that one is preferred.

The function is exposed in RNA as `fcurve.keyframe_points.deduplicate()`

This commit also introduces a new function `BKE_fcurve_bezt_shrink(fcu,
new_totvert);` that can reallocate the `bezt` array to ensure removed
keys no longer take up memory.

The RNA function `fcurve.update()` currently performs two steps, which
are now exposed to RNA as well, as `keyframe_points.sort()` and
`keyframe_points.handles_recalc()`. This is so that Python code can
sort, deduplicate, and then recalculate the handles only once (calling
`update` + `deduplicate` would do the latter twice).

In Blender 4.0 the deduplication will also be part of `fcurve.update()`,
see #107126.

Reviewed on https://projects.blender.org/blender/blender/pulls/107089

scripts/modules/bpy_extras/anim_utils.py

@@ -571,4 +571,8 @@ class KeyframesCo:
             keyframe_points.foreach_set("co", co_buffer)
             keyframe_points.foreach_set("interpolation", ipo_buffer)
 
-            fcurve.update()
+            # TODO: in Blender 4.0 the next lines can be replaced with one call to `fcurve.update()`.
+            # See https://projects.blender.org/blender/blender/issues/107126 for more info.
+            keyframe_points.sort()
+            keyframe_points.deduplicate()
+            keyframe_points.handles_recalc()