3.5.1: reintroduced a suspend mode in WaitForNextFrame()

[ivan-mogilko]

The problem is briefly mentioned here: #1287 (comment)

Previously the engine was using allegro timers that were suspended by default when the player switched away from the game. That stopped game from updating and redrawing itself.

The commits f2ab428 and 15489ff broke that behavior. Here we reintroduce it, now implementing the timer lock mechanic directly in the engine. This is a bit naive implementation but somewhat in line with the older code (e.g. see https://github.com/adventuregamestudio/ags/blob/release-3.5.0/Engine/ac/timer.cpp#L40).

[ivan-mogilko]

In regards to 3.6.0 (SDL2-based), unfortunately this fix is not enough is not suitable.

In allegro version the "switch out" callback is run on a separate thread, and therefore can intercept a game update thread at the moment between two updates (frames). It then allows to suspend and continue game update thread as necessary.

In its current state in 3.6.0 all sdl events are processed on the same thread as game update, which means that if "switch out" event is handled the process will still be inside a game frame function. Therefore we cannot suspend there - it will simply never recover.

To fix this we need to have at least relevant sdl events processed on a separate thread. Perhaps, ideally the events are processed on main thread and the game update has its own. In any case a larger rewrite is necessary.

[ivan-mogilko]

In regards to my above comment, I found it's actually possible to solve this in 3.6.0 with a minimal addition, which hopefully will work for now.