make pipeline reports delta_time to imgui

[ruby3141]

According to imgui's custom backend implementation exhibit
and first-party backend implementations,
io.DeltaTime is "usually" meant to be updated by backend code.

(you can check platform backend impls of ocornut/imgui for example)

This PR solves issue #186.

[veeenu]

I'm not super sure what happens when the delta time ends up being Duration::ZERO, but it seems to be working. LGTM. Thank you for your contribution!

[ruby3141]

imgui prohibits IO.DeltaTime of 0 when it's not in its first frame,
but Duration::ZERO will be boundary checked by imgui::Io::update_delta_time() into f32::MIN_POSITIVE seconds.
/~https://github.com/ocornut/imgui/blob/a1566c5e1ba22755c359e3079f5f25ab53f1eafb/imgui.cpp#L9784
/~https://github.com/imgui-rs/imgui-rs/blob/cb8beeb74cceec787275ec3e121c0068ba08fbf7/imgui/src/io.rs#L435-L437

I've considered Duration::from_nanos(1) instead of Duration::ZERO, but turns out it was too large.

f32::MIN_POSITIVE seconds is small enough not to actually increment global timer(which is f64 btw) while being positive.
it is about 0.000000000000000000000000000000000000011754944 seconds, or 0.00000001 quectoseconds,
and when added into f64 global timer, which is usually larger than 1, it just rounded down into zero.
(It has about 30 zeros too many. In f64, the very next value after 1 is 1.0000000000000002.)

[veeenu]

That's great, thank you!