Fix timer with zero duration

[CrazyRoka]

Objective

Timer with zero Duration panics at tick() because of division by zero. This PR Fixes #8463 .

Solution

• Handle division by zero separately with checked_div and checked_rem.

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Changelog

• Replace division with checked_div. Set times_finished_this_tick to u32::MAX when duration is zero.
• Set elapsed to Duration::ZERO when timer duration is zero.
• Set percent to 1.0 when duration is zero.
• times_finished_this_tick is not used anywhere, that's why this change will not affect other parts of the project.
• times_finished_this_tick is set to 0 after reset() and before first tick() call.

[alice-i-cecile]

These fixes look correct: nice work handling this robustly.

The other alternative would be to simply make it impossible to construct a timer with zero duration. It's not entirely clear what behavior would be best there though: panicking on construction simply moves the problem. We could enforce this with NonZero types, but again, constructing those usually involves an unwrap or other surprising failure.

[alice-i-cecile]

Ultimately I think this is the least problematic way to fix this panic.

[mockersf]

I would prefer to make the issue impossible by types, as this means the check will happen several times. Someone doing lots of timers and calling percent on every frame may notice an impact... but we can revisit that then