image-source: Defer update of slideshow source

Defers the update of the slideshow source until the user has pressed OK
or exited the properties dialog.  This ensures images aren't reloaded
every time the user adjusts a setting.

plugins/image-source/image-source.c

@@ -230,6 +230,8 @@ static obs_properties_t *image_source_properties(void *data)
 
 	obs_properties_t *props = obs_properties_create();
 
+	obs_properties_set_flags(props, OBS_PROPERTIES_DEFER_UPDATE);
+
 	if (s && s->file && *s->file) {
 		const char *slash;