Cleanup: Remove scene frame macros (CFRA et al.)

Removes the following macros for scene/render frame values:
- `CFRA`
- `SUBFRA`
- `SFRA`
- `EFRA`

These macros don't add much, other than saving a few characters when typing.
It's not immediately clear what they refer to, they just hide what they
actually access. Just be explicit and clear about that.
Plus these macros gave read and write access to the variables, so eyesores like
this would be done (eyesore because it looks like assigning to a constant):
```
CFRA = some_frame_nbr;
```

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D15311

source/blender/editors/animation/anim_motion_paths.c

@@ -391,7 +391,7 @@ void animviz_calc_motionpaths(Depsgraph *depsgraph,
     return;
   }
 
-  const int cfra = CFRA;
+  const int cfra = scene->r.cfra;
   int sfra = INT_MAX, efra = INT_MIN;
   switch (range) {
     case ANIMVIZ_CALC_RANGE_CURRENT_FRAME:
@@ -485,7 +485,7 @@ void animviz_calc_motionpaths(Depsgraph *depsgraph,
             sfra,
             efra,
             efra - sfra + 1);
-  for (CFRA = sfra; CFRA <= efra; CFRA++) {
+  for (scene->r.cfra = sfra; scene->r.cfra <= efra; scene->r.cfra++) {
     if (range == ANIMVIZ_CALC_RANGE_CURRENT_FRAME) {
       /* For current frame, only update tagged. */
       BKE_scene_graph_update_tagged(depsgraph, bmain);
@@ -496,14 +496,14 @@ void animviz_calc_motionpaths(Depsgraph *depsgraph,
     }
 
     /* perform baking for targets */
-    motionpaths_calc_bake_targets(targets, CFRA);
+    motionpaths_calc_bake_targets(targets, scene->r.cfra);
   }
 
   /* reset original environment */
   /* NOTE: We don't always need to reevaluate the main scene, as the depsgraph
    * may be a temporary one that works on a subset of the data.
    * We always have to restore the current frame though. */
-  CFRA = cfra;
+  scene->r.cfra = cfra;
   if (range != ANIMVIZ_CALC_RANGE_CURRENT_FRAME && restore) {
     motionpaths_calc_update_scene(depsgraph);
   }