Fix 100718 NLA Hold Forward Inconsistency

Problem:
For the Action Track, 'extrapolation=Hold Forward' behaves the same as
'Hold'. Animators expect 'Hold Forward' to behave consistently with the
rest of the NLA system but it does not.

Source of the problem:
Pre-patch, the problematic behavior was implemented as a special case
(see: animsys_create_action_track_strip() in anim_sys.c). It was added
added by commit 2826c2be54. The fix
effectively removes the special handling while keeping the core intent
of the original commit, to have the Action Track evaluate fcurve
extrapolation properly.

Solution:
For the Action Track, we now properly treat extrapolation Hold_Forward
just like the rest of the NLA system (strips): as bounded on the left
and infinite on the right. In (anim_sys.c) nlastrips_ctime_get_strip(),
an additional boundary check is required since the original commit
assumed the only possible case, when NLASTRIP_FLAG_NO_TIME_MAP is
enabled, is effectively extrapolation=HOLD.

TODO: upload quick test blend file
TODO: maybe show video of before/after but I think the test blend file will be fine.

@@ -3219,0 +3224,4 @@
+   *
+   * Must set NLASTRIP_FLAG_NO_TIME_MAP, so Action Track fcurve evaluation extends beyond its
+   * keyframe bounds. */
+  r_action_strip->flag |= NLASTRIP_FLAG_USR_INFLUENCE | NLASTRIP_FLAG_NO_TIME_MAP;

@@ -994,0 +995,4 @@
+     * fcurve extrapolation isn't clamped to the keyframe bounds. */
+    bool in_range = IN_RANGE_INCL(ctime, strip->start, strip->end);
+    if (strip->flag & NLASTRIP_FLAG_NO_TIME_MAP) {
+      if (ELEM(strip->extendmode, NLASTRIP_EXTEND_HOLD)) {