Python: Add new `annotation_pre` & `annotation_post` handlers

Annotation tool is used as a general mark tool for many add-ons. To be able to detect when an annotation is done is very handy to integrate the annotation tool in add-ons and other studio workflows.

The  new callback names are:  `annotation_pre`  and `annotation_post`

Both callbacks are exposed via the Python module `bpy.app.handlers`

Example use:

```
import bpy

def annotation_starts(gpd):
    print("Annotation starts")

def annotation_done(gpd):
    print("Annotation done")

bpy.app.handlers.annotation_pre.clear()
bpy.app.handlers.annotation_pre.append(annotation_starts)
bpy.app.handlers.annotation_post.clear()
bpy.app.handlers.annotation_post.append(annotation_done)
```

Note: The handlers are called for any annotation tool, including eraser.

Reviewed By: campbellbarton

Differential Revision: https://developer.blender.org/D14221

source/blender/blenkernel/BKE_callbacks.h

@@ -95,6 +95,8 @@ typedef enum {
   BKE_CB_EVT_LOAD_FACTORY_USERDEF_POST,
   BKE_CB_EVT_LOAD_FACTORY_STARTUP_POST,
   BKE_CB_EVT_XR_SESSION_START_PRE,
+  BKE_CB_EVT_ANNOTATION_PRE,
+  BKE_CB_EVT_ANNOTATION_POST,
   BKE_CB_EVT_TOT,
 } eCbEvent;
 

source/blender/editors/gpencil/annotate_paint.c

@@ -22,6 +22,7 @@
 
 #include "PIL_time.h"
 
+#include "BKE_callbacks.h"
 #include "BKE_colortools.h"
 #include "BKE_context.h"
 #include "BKE_global.h"
@@ -1509,6 +1510,9 @@ static void annotation_paint_initstroke(tGPsdata *p,
   Scene *scene = p->scene;
   ToolSettings *ts = scene->toolsettings;
 
+  /* Call to the annotation pre handler to notify python the annotation starts. */
+  BKE_callback_exec_id_depsgraph(p->bmain, &p->gpd->id, p->depsgraph, BKE_CB_EVT_ANNOTATION_PRE);
+
   /* get active layer (or add a new one if non-existent) */
   p->gpl = BKE_gpencil_layer_active_get(p->gpd);
   if (p->gpl == NULL) {
@@ -1675,6 +1679,9 @@ static void annotation_paint_strokeend(tGPsdata *p)
     annotation_stroke_newfrombuffer(p);
   }
 
+  /* Call to the annotation post handler to notify python the annotation is done. */
+  BKE_callback_exec_id_depsgraph(p->bmain, &p->gpd->id, p->depsgraph, BKE_CB_EVT_ANNOTATION_POST);
+
   /* clean up buffer now */
   annotation_session_validatebuffer(p);
 }

source/blender/python/intern/bpy_app_handlers.c

@@ -64,6 +64,8 @@ static PyStructSequence_Field app_cb_info_fields[] = {
     {"load_factory_preferences_post", "on loading factory preferences (after)"},
     {"load_factory_startup_post", "on loading factory startup (after)"},
     {"xr_session_start_pre", "on starting an xr session (before)"},
+    {"annotation_pre", "on drawing an annotation (before)"},
+    {"annotation_post", "on drawing an annotation (after)"},
 
 /* sets the permanent tag */
 #define APP_CB_OTHER_FIELDS 1