NLA: Keyframe Remap Through Upper Strips

Differential Revision: https://developer.blender.org/D10504

include D9925: NLA Tweak Mode: Evaluate Upper Stack

- ghost fcurve drawing uses Show Extrapolation setting

- Overlooked remap_domain writing, now sets remap_domain on output even when succeeds. This didn't have any effect for user as the input blended_snapshot and output upper_snapshot were always the same.

- rebase and minor cleanup

NLA: Extract nlasnapshot_blend_get_inverted_upper_snapshot()

Extracts //nlasnapshot_blend_get_inverted_upper_snapshot()// from //BKE_animsys_nla_remap_keyframe_values()//

- This introduces a new struct member: `NlaEvalChannelSnapshot->invertible` which marks values which have successfully been inverted and can be further used for solving.
- //nlasnapshot_mark_all_invertible()// is unused in this patch but seemed generally useful when using snapshots for remapping. {D8867} would be able to make use of it.
- //nlasnapshot_blend_get_inverted_upper_snapshot()// has a variant //nlasnapshot_blend()// from {D10220}, but this patch doesn't depend on it at all. A third variant will later be added //nlasnapshot_blend_get_inverted_lower_snapshot()//. Altogether, these three functions allow solving for any of (lower_snapshot, upper_snapshot, blended_snapshot) given the other two.

No user functional changes

Differential Revision: https://developer.blender.org/D10222

minor cleanup

Fix: NLA Blend Domain Enable All Typo

Introduced by my recent commit: rBc48360c2559a

Accidentally used 4 instead of `necs->length`.

Fix: NLA Blends Non-Animated Upper Channel Values

Issue introduced by my commit: rB40b7929cc040

**User-level Problem**:
The issue resulted in a full-replace upper strip with only a Z location
channel full-replacing the XY location channels of the lower stack.
replaced to default. The expected behavior is that only the Z location
channel is affected.

**Technical-level Problem**:
Before the problematic commit, fcurves were blended as they were read.
So only existing animated channels would blend. My recent commit
changed the process to read all fcurve values into an isolated
upper_snapshot then blend with the lower stack. There is no data stored
to know whether the upper snapshot channel values were sampled from
fcurves or were default values. Only those sampled from fcurves should
be blended.

**Solution**:
Added a `blend_domain` bitmask member to NlaEvalChannelSnapshot.
The blending function only blends values within the `blend_domain`.
Sampled fcurve values are now marked as within the `blend_domain`.
We also now always copy the lower snapshot to the result snapshot which
only matters when they aren't the same. Currently, it's always the same
so the change is more for future unseen cases.

Reviewed By: sybren, #animation_rigging

Differential Revision: https://developer.blender.org/D10339

Graph Editor: FCurve Extrapolation Visibility T76472

Adds toggle to graph editor (View->Show Extrapolation). When disabled, then fcurves only draw over the keyframe range. For baked fcurves, the range is all sampled points. For ghost fcurves, extraplation is always drawn since the sampled points may include extrapolated points of the original fcurve.

Show Extrapolation is the default.
___

**Technical Notes**:
- `draw_fcurve_curve()`: There are now explicit checks for floating point poor accuracy cases. Without them, then there would be visual flickering when no extrapolation is drawn and there is a discontinuity at the end (due to cyclic modifier). This removes the need for the `+samplefreq` in `etime = v2d->curxmax + samplefreq`. It also removes the need to do so on `fcu_start/fcu_end`.
- `draw_fcurve_curve()`: Besides the previous note, the only other meaningful change is the `if(!draw_extrapolation)` branch which just changes `stime/etime` to match the keyframe bounds instead.

Differential Revision: https://developer.blender.org/D10442