Fix race condition for Paragraph and Editor in the renderer

[srussvoll]

I'm working on the bevy_iced plugin for Bevy that integrates Iced into Bevy:

• Bevy 0.16 support, and fixed blinking text and ability to request redraws tasgon/bevy_iced#35

Since Bevy is using pipelined rendering and Iced assumes non-pipelined rendering, some things don't work. For instance, when typing into a text input widget, the text will often disappear for a frame when a letter is typed resulting in blinking, and is reported in #2318. The issue is that there is a race between the renderer reading the weakly referenced text and the destroying of that text, and is described in more detail here: #2670 (comment). I started a thread in Discord to discuss this problem more generically.

This PR fixes the race condition that causes the flashing text.

The following PR is also required to get bevy_iced to work with the main branch of iced:

• Let the user of iced_wgpu::Renderer manually finish/recall the staging belt #2693