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>
Merge: Mnit fps

Factorize the frame-rate control of mnit applications in a dedicated module in lib.

Pull-Request: #505
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Romain Chanoir <chanoir.romain@courrier.uqam.ca>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>