Nla: Rename NlaEvalChannel->valid to domain

For term consistency with usage.

The clarity is more for consistency with the nla domain() processing
function names and the core struct member name it stores the results
in, "valid". The name "domain", which implies a function can operate
on it, seems more natural than "valid", which implies something is
wrong if false.

No functional changes.

Reviewed by: sybren

Differential Revision: http://developer.blender.org/D9692

source/blender/blenkernel/nla_private.h

@@ -112,8 +112,8 @@ typedef struct NlaEvalChannel {
   struct NlaEvalChannel *next_blend;
   NlaEvalChannelSnapshot *blend_snapshot;
 
-  /* Mask of array items controlled by NLA. */
-  NlaValidMask valid;
+  /* Associated with the RNA property's value(s), marks which elements are affected by NLA. */
+  NlaValidMask domain;
 
   /* Base set of values. */
   NlaEvalChannelSnapshot base_snapshot;