T77086 Animation: Passing Dependency Graph to Drivers

Custom driver functions need access to the dependency graph that is
triggering the evaluation of the driver. This patch passes the
dependency graph pointer through all the animation-related calls.

Instead of passing the evaluation time to functions, the code now passes
an `AnimationEvalContext` pointer:

```
typedef struct AnimationEvalContext {
  struct Depsgraph *const depsgraph;
  const float eval_time;
} AnimationEvalContext;
```

These structs are read-only, meaning that the code cannot change the
evaluation time. Note that the `depsgraph` pointer itself is const, but
it points to a non-const depsgraph.

FCurves and Drivers can be evaluated at a different time than the
current scene time, for example when evaluating NLA strips. This means
that, even though the current time is stored in the dependency graph, we
need an explicit evaluation time.

There are two functions that allow creation of `AnimationEvalContext`
objects:

- `BKE_animsys_eval_context_construct(Depsgraph *depsgraph, float
  eval_time)`, which creates a new context object from scratch, and
- `BKE_animsys_eval_context_construct_at(AnimationEvalContext
  *anim_eval_context, float eval_time)`, which can be used to create a
  `AnimationEvalContext` with the same depsgraph, but at a different
  time. This makes it possible to later add fields without changing any
  of the code that just want to change the eval time.

This also provides a fix for T75553, although it does require a change
to the custom driver function. The driver should call
`custom_function(depsgraph)`, and the function should use that depsgraph
instead of information from `bpy.context`.

Reviewed By: brecht, sergey

Differential Revision: https://developer.blender.org/D8047

source/blender/editors/interface/interface.c

@@ -44,6 +44,7 @@
 
 #include "BLI_utildefines.h"
 
+#include "BKE_animsys.h"
 #include "BKE_context.h"
 #include "BKE_idprop.h"
 #include "BKE_main.h"
@@ -1733,6 +1734,7 @@ void UI_block_end_ex(const bContext *C, uiBlock *block, const int xy[2], int r_x
   wmWindow *window = CTX_wm_window(C);
   Scene *scene = CTX_data_scene(C);
   ARegion *region = CTX_wm_region(C);
+  Depsgraph *depsgraph = CTX_data_depsgraph_pointer(C);
   uiBut *but;
 
   BLI_assert(block->active);
@@ -1761,7 +1763,9 @@ void UI_block_end_ex(const bContext *C, uiBlock *block, const int xy[2], int r_x
       }
     }
 
-    ui_but_anim_flag(but, (scene) ? scene->r.cfra : 0.0f);
+    const AnimationEvalContext anim_eval_context = BKE_animsys_eval_context_construct(
+        depsgraph, (scene) ? scene->r.cfra : 0.0f);
+    ui_but_anim_flag(but, &anim_eval_context);
     ui_but_override_flag(CTX_data_main(C), but);
     if (UI_but_is_decorator(but)) {
       ui_but_anim_decorate_update_from_flag(but);