MultiEditorViewController: Reload the current editor during export process

[twstokes]

Description

This PR reloads the current editor when exporting from a MultiEditorViewController so that layers don't disappear if the user cancels the process. This currently affects WPiOS and appears to have been a problem in the past fixed by this PR. However, that fix doesn't seem to have the same effect anymore.

What this PR aims to do:

• Only reload the currently selected (and visible, for WPiOS) editor rather than all exported editors
• Reduce memory consumption and retention during the export process

⚠️ I'm unsure how this may affect the Tumblr or any other clients. Since the original fix was for WPiOS I'm inclined to think Tumblr won't be negatively affected, but I'm OK with implementing a more backwards-compatible fix if we think it's a risk.

Related issues:

• Stories: Scale to fill rendering + post-export overlays bug fixes #106
• WPiOS Crashed When Adding Media to a Story Post wordpress-mobile/WordPress-iOS#18550)

Testing

1. Run WPiOS from this PR
2. Create a new Story post
3. Add media from the device
4. Add a text layer
5. Tap the arrow at the top right to start the export / posting process
6. Cancel the posting process by swiping the publishing sheet down
7. Observe that the text from step 4 is retained

Before	After
Before.MP4	After.mp4

[twstokes]

@afterxleep, @bjtitus here's the second change related to wordpress-mobile/WordPress-iOS#18967. If you could give this a look I'd appreciate it! 🙇

[bjtitus]

This looks good to me and I think it is fine for the Tumblr app as well. I was hoping we might get the Buildkite integration in before merging this, but it should be good to go!

[twstokes]

This looks good to me and I think it is fine for the Tumblr app as well.

Thanks @bjtitus for checking it out! 🙇

I was hoping we might get the Buildkite integration in before merging this, but it should be good to go!

If tagging and pushing 1.4.3 (after #140 is reviewed) to CocoaPods complicates anything, I understand if you all want to get the Buildkite integration going first.