PyAPI: Store PyInstances for ID's

This means once an ID is created,
it will keep using the same PyObject instance.

This has some advantages:
- Avoids unnecessary re-creation of instances on UI poll / redraw.
- Accessing free'd ID's gives an exception instead of crashing.
  (long standing annoyance!, though this only applies to ID's
   and not yet other data that uses the ID's - vertices for eg).
- Allows using instance comparison (a little faster).

Note that the instances won't be kept between undo.

source/blender/blenloader/intern/readfile.c

@@ -2221,6 +2221,7 @@ static void direct_link_id(FileData *fd, ID *id)
 		/* this case means the data was written incorrectly, it should not happen */
 		IDP_DirectLinkGroup_OrFree(&id->properties, (fd->flags & FD_FLAGS_SWITCH_ENDIAN), fd);
 	}
+	id->py_instance = NULL;
 }
 
 /* ************ READ CurveMapping *************** */

source/blender/makesdna/DNA_ID.h

@@ -143,6 +143,7 @@ typedef struct ID {
 	int us;
 	int icon_id;
 	IDProperty *properties;
+	void *py_instance;
 } ID;
 
 /**

source/blender/makesdna/DNA_object_types.h

@@ -299,15 +299,15 @@ typedef struct Object {
 
 	struct FluidsimSettings *fluidsimSettings; /* if fluidsim enabled, store additional settings */
 
-	/* Runtime valuated curve-specific data, not stored in the file */
-	struct CurveCache *curve_cache;
-
 	struct DerivedMesh *derivedDeform, *derivedFinal;
 	uint64_t lastDataMask;   /* the custom data layer mask that was last used to calculate derivedDeform and derivedFinal */
 	uint64_t customdata_mask; /* (extra) custom data layer mask to use for creating derivedmesh, set by depsgraph */
 	unsigned int state;			/* bit masks of game controllers that are active */
 	unsigned int init_state;	/* bit masks of initial state as recorded by the users */
 
+	/* Runtime valuated curve-specific data, not stored in the file */
+	struct CurveCache *curve_cache;
+
 	ListBase gpulamp;		/* runtime, for glsl lamp display only */
 	ListBase pc_ids;
 	ListBase *duplilist;	/* for temporary dupli list storage, only for use by RNA API */

source/blender/makesdna/DNA_scene_types.h

@@ -1720,12 +1720,12 @@ typedef struct Scene {
 	/* Physics simulation settings */
 	struct PhysicsSettings physics_settings;
 
-	/* Movie Tracking */
-	struct MovieClip *clip;			/* active movie clip */
-
 	uint64_t customdata_mask;	/* XXX. runtime flag for drawing, actually belongs in the window, only used by BKE_object_handle_update() */
 	uint64_t customdata_mask_modal; /* XXX. same as above but for temp operator use (gl renders) */
 
+	/* Movie Tracking */
+	struct MovieClip *clip;			/* active movie clip */
+
 	/* Color Management */
 	ColorManagedViewSettings view_settings;
 	ColorManagedDisplaySettings display_settings;

source/blender/makesdna/DNA_text_types.h

@@ -61,8 +61,8 @@ typedef struct Text {
 	char *undo_buf;
 	int undo_pos, undo_len;
 	
-	void *compiled;
 	double mtime;
+	void *compiled;
 } Text;
 
 #define TXT_TABSIZE	4

source/blender/makesrna/intern/rna_ID.c

@@ -394,6 +394,14 @@ static void rna_ID_animation_data_free(ID *id, Main *bmain)
 	DEG_relations_tag_update(bmain);
 }
 
+#ifdef WITH_PYTHON
+void **rna_ID_instance(PointerRNA *ptr)
+{
+	ID *id = (ID *)ptr->data;
+	return &id->py_instance;
+}
+#endif
+
 static void rna_IDPArray_begin(CollectionPropertyIterator *iter, PointerRNA *ptr)
 {
 	IDProperty *prop = (IDProperty *)ptr->data;
@@ -1071,6 +1079,10 @@ static void rna_def_ID(BlenderRNA *brna)
 	                                "Tag the ID to update its display data, "
 	                                "e.g. when calling :class:`bpy.types.Scene.update`");
 	RNA_def_enum_flag(func, "refresh", update_flag_items, 0, "", "Type of updates to perform");
+
+#ifdef WITH_PYTHON
+	RNA_def_struct_register_funcs(srna, NULL, NULL, "rna_ID_instance");
+#endif
 }
 
 static void rna_def_library(BlenderRNA *brna)

source/blender/makesrna/intern/rna_internal.h

@@ -221,6 +221,7 @@ void rna_ID_name_set(struct PointerRNA *ptr, const char *value);
 struct StructRNA *rna_ID_refine(struct PointerRNA *ptr);
 struct IDProperty *rna_ID_idprops(struct PointerRNA *ptr, bool create);
 void rna_ID_fake_user_set(struct PointerRNA *ptr, int value);
+void **rna_ID_instance(PointerRNA *ptr);
 struct IDProperty *rna_PropertyGroup_idprops(struct PointerRNA *ptr, bool create);
 void rna_PropertyGroup_unregister(struct Main *bmain, struct StructRNA *type);
 struct StructRNA *rna_PropertyGroup_register(struct Main *bmain, struct ReportList *reports, void *data,