Let the user of iced_wgpu::Renderer manually finish/recall the staging belt

[Azorlogh]

The iced integration for bevy needs to let Bevy take care of submitting the queue, so it can't call Renderer::present. But it still needs to call finish/recall on the staging belt.
This adds two methods Renderer::staging_belt_finish and Renderer::staging_belt_recall to let the user do it manually.

[srussvoll]

Bevy 0.15 support has been completed in tasgon/bevy_iced#34, and is blocked by this PR. What is the status of this PR? Are there any changes needed to get this through?

[Azorlogh]

I updated the PR. Since recent changes the only thing left is the ability to manually finish & recall the staging belt.

[Azorlogh]

Actually there is a second change needed 😅 We need to be able to call Renderer::draw too. I made it public

[hecrj]

Why does present not work exactly? Why does Bevy need to control all Queue submissions?

[Azorlogh]

@hecrj In order to integrate to bevy's render graph, we need to append our commands to the Queue it provides us. It only submit the queue after the whole graph finished running.
If iced submits the work to its own queue, then everything else will be drawn on top of its UI