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/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)"},