lib/mnit: introduce mnit_fps so each app does not need to play with clocks

Signed-off-by: Jean Privat <jean@pryen.org>
mnit: update examples to use maximim_fps

Signed-off-by: Jean Privat <jean@pryen.org>
mnit_fps: add current_fps

Always useful.

Signed-off-by: Jean Privat <jean@pryen.org>