Request Redraw only if really needed. Fixes #277

[Maximetinu]

Hello,

I had this problem that I described at this issue:

• How can I stop bevy_egui from sending RequestRedraw whenever I move the mouse? #277

where it is a blocker for my use case that bevy_egui sends RequestRedraw events, even when I'm not painting any EGUI window.

So, this PR fixes the process_output_system so that it doesn't emit that event when nothing is being painted.

It will keep working as usual when EGUI is painting.

---

On another note:

I'm not sure that RequestRedraw is really needed. If the UpdateMode of the Bevy's WinitSettings resource is UpdateMode::Continuous, Bevy will redraw anyway. If it is UpdateMode::Reactive or UpdateMode::ReactiveLowPower, both of them should answer to Window events, like moving the cursor (see here in bevy_winit)

However, I'm not removing it in this PR to be cautious and not reintroduce previous bugs like

• Fix requested repaints not causing Bevy to redraw. #240
• Using WinitSettings::desktop_app() does not update egui UI #239
• Reactive mode support #176

[mvlabat]

Hi, thank you for the PR! Lgtm, apologies for taking that long to review it