Deterministic timer simulation for shrine messages in demo mode

[StephenCWills]

Solves one of the issues listed in #2745.

Dialogs playback is realtime, if the player did not interrupt it (or playback is slower then playtime) the dialog will stay open for a different duration

When recording a demo or playing it back, the game simulates SDL_GetTicks() based on the number of game ticks that have elapsed since the start of the demo, assuming 20 fps. When recording or playing back at faster speeds, this means that shrine messages will disappear faster and dialog will move faster than the corresponding audio.

[StephenCWills]

Now that I'm reading it again, I suppose the issue I mentioned earlier was probably referring to speech text, like if you talk to Ogden after starting a new game. This doesn't resolve that, but it does work for shrines. Never mind, it was very easy to include dialog as well.

I'm not sure I like the name of the GetTicks() function. And I definitely don't like having devilution::GetTicks() and demo::GetTicks(). I'm thinking about whether to change one or both of these function names. Maybe.. demo::SimulateTicksAtNormalSpeed()?

EDIT: I think I'm pretty happy with devilution::GetMillisecondsSinceStartup() and demo::SimulateMillisecondsSinceStartup(). Unfortunately, the thing it still doesn't really convey is why you would use this instead of SDL_GetTicks() directly.