Blender Git Loki

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