Add a specific parser for these, which can be accessed as gcoder.S/P

This also reverts to wx.SHAPED for the gviz packing, and specifies a minimum
size with the correct aspect ratio.

The print thread must always respawn the sender thread when ending (the change
introduced by 9c55957e was checking stop_send_thread, which is set to False by
_start_sender, so it never got it). Instead of adding more logic, let's just
always respawn the thread and kill it immediately.