util: Lower the priority of the helper spawn
The default spawn method of the context, gets scheduled with G_PRIORITY_DEFAULT (0), which takes priority over GTK's GDK_PRIORITY_REDRAW which has a value of 120.
#define GDK_PRIORITY_REDRAW (G_PRIORITY_HIGH_IDLE + 20)
This causes all the futures we spawn to take priority over redraw and potentially miss/drop frames.
Switch the default priority we use to G_PRIORITY_DEFAULT_IDLE (200) so that it will always be scheduled after GDK_PRIORITY_REDRAW