Add callback for starting a render-job

We had complete/cancel, but no matching init for rendering,
render_pre/post callbacks aren't always usable.

source/blender/blenlib/BLI_callbacks.h

@@ -29,12 +29,20 @@ struct bContext;
 struct Main;
 struct ID;
 
+/**
+ * Common suffix uses:
+ * - ``_PRE/_POST``:
+ *   For handling discrete non-interactive events.
+ * - ``_INIT/_COMPLETE/_CANCEL``:
+ *   For handling jobs (which may in turn cause other handlers to be called).
+ */
 typedef enum {
 	BLI_CB_EVT_FRAME_CHANGE_PRE,
 	BLI_CB_EVT_FRAME_CHANGE_POST,
 	BLI_CB_EVT_RENDER_PRE,
 	BLI_CB_EVT_RENDER_POST,
 	BLI_CB_EVT_RENDER_STATS,
+	BLI_CB_EVT_RENDER_INIT,
 	BLI_CB_EVT_RENDER_COMPLETE,
 	BLI_CB_EVT_RENDER_CANCEL,
 	BLI_CB_EVT_LOAD_PRE,

source/blender/python/intern/bpy_app_handlers.c

@@ -49,6 +49,7 @@ static PyStructSequence_Field app_cb_info_fields[] = {
 	{(char *)"render_pre",        (char *)"Callback list - on render (before)"},
 	{(char *)"render_post",       (char *)"Callback list - on render (after)"},
 	{(char *)"render_stats",      (char *)"Callback list - on printing render statistics"},
+	{(char *)"render_init",       (char *)"Callback list - on initialization of a render job"},
 	{(char *)"render_complete",   (char *)"Callback list - on completion of render job"},
 	{(char *)"render_cancel",     (char *)"Callback list - on canceling a render job"},
 	{(char *)"load_pre",          (char *)"Callback list - on loading a new blend file (before)"},

source/blender/render/intern/source/pipeline.c

@@ -2779,6 +2779,8 @@ void RE_SetReports(Render *re, ReportList *reports)
 void RE_BlenderFrame(Render *re, Main *bmain, Scene *scene, SceneRenderLayer *srl, Object *camera_override,
                      unsigned int lay_override, int frame, const bool write_still)
 {
+	BLI_callback_exec(re->main, (ID *)scene, BLI_CB_EVT_RENDER_INIT);
+
 	/* ugly global still... is to prevent preview events and signal subsurfs etc to make full resol */
 	G.is_rendering = true;
 	
@@ -2953,6 +2955,8 @@ void RE_BlenderAnim(Render *re, Main *bmain, Scene *scene, Object *camera_overri
 	int cfrao = scene->r.cfra;
 	int nfra, totrendered = 0, totskipped = 0;
 	
+	BLI_callback_exec(re->main, (ID *)scene, BLI_CB_EVT_RENDER_INIT);
+
 	/* do not fully call for each frame, it initializes & pops output window */
 	if (!render_initialize_from_main(re, &rd, bmain, scene, NULL, camera_override, lay_override, 0, 1))
 		return;