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/physics/physics_fluid.c

@@ -256,8 +256,8 @@ static void fluid_bake_sequence(FluidJob *job)
   frame = is_first_frame ? fds->cache_frame_start : (*pause_frame);
 
   /* Save orig frame and update scene frame. */
-  orig_frame = CFRA;
-  CFRA = frame;
+  orig_frame = scene->r.cfra;
+  scene->r.cfra = frame;
 
   /* Loop through selected frames. */
   for (; frame <= fds->cache_frame_end; frame++) {
@@ -280,7 +280,7 @@ static void fluid_bake_sequence(FluidJob *job)
       *(job->progress) = progress;
     }
 
-    CFRA = frame;
+    scene->r.cfra = frame;
 
     /* Update animation system. */
     ED_update_for_newframe(job->bmain, job->depsgraph);
@@ -293,7 +293,7 @@ static void fluid_bake_sequence(FluidJob *job)
   }
 
   /* Restore frame position that we were on before bake. */
-  CFRA = orig_frame;
+  scene->r.cfra = orig_frame;
 }
 
 static void fluid_bake_endjob(void *customdata)