The goal is to control the rate of capture while in screencast, as it
can represent a performance issue and can cause input lag and the
feeling of having a laggy mouse.
This commit addresses the issue reported in #66.
The code measures the time elapsed to make a single screen capture, and
calculates how much to wait for the next capture to achieve the targeted
frame rate. To delay the capturing of the next frame, the code
introduces timers into the event loop based on the event loop in
https://github.com/emersion/mako
Added a command-line argument and an entry in the config file as well
for the max FPS. The default value is 0, meaning no rate control.
Added code to measure the average FPS every 5 seconds and print it with
DEBUG level.
