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,