Make picopolis work

[mibus]

I wanted to try out Picopolis - https://www.lexaloffle.com/bbs/?tid=29590 - and these patches are enough to begin playing.

The two commits implement flip() [using a function pointer and data pointer, not unlike glib-style userdata callbacks] and re-fetch _update() and friends from Lua (since Picopolis has _update and _draw as function pointers).