luaengine: add HUD-like capabilities

These commits add a way to let LUA scripts register a callback
to be invoked before rendering each frame.
This callback typically makes use of other provided methods to draw
custom graphics on top of each frame.

The idea is to let LUA script provide HUD-like features, mostly used for
trainings and TAS runs development.

Signed-off-by: Luca Bruno lucab@debian.org