Blender Git Loki

NLA: Extract ..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->remap_domain` and marks which values of
`blended_snapshot` are processed for remapping/used-for-inverting.
Effectively, it marks which values have successfully been remapped and
can be further used for remapping.

`nlasnapshot_blend_get_inverted_upper_snapshot()`:
output snapshot `r_upper_snapshot` has each channel's `remap_domain`
written to which effectively marks the successfully remapped values.
The only reason a value is not in the remap domain is if inversion
failed or it wasn't marked to be remapped.

`..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 `..get_inverted_lower_snapshot()`.
Altogether, these three functions allow solving for any of
(lower_snapshot, upper_snapshot, blended_snapshot) given the other two.
The function `..get_inverted_lower_snapshot()` will also similarly
process the remap domain of the blended and lower snapshot.

added assertions within `nlasnapshot_blend()` and
`..get_inverted_upper_snapshot()` to future proof branches dealing with
blendmode and mixmodes. (suggested by sybren)

No user functional changes

Reviewed By: sybren

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