Fix broken previews after cropping

[lakesare]

Fixes #4163, possibly fixes #4183

Before

{ waitForThumbnailsBeforeUpload: false } + edit the image => thumbnail gets properly generated
{ waitForThumbnailsBeforeUpload: true } + edit the image => NO thumbnail

After

{ waitForThumbnailsBeforeUpload: false } + edit the image => thumbnail gets properly generated
{ waitForThumbnailsBeforeUpload: true } + edit the image => thumbnail gets properly generated

---

Comments

I think our waitForThumbnailsBeforeUpload + lazy options for ThumbnailGenerator are kind of weird.

It's nice to have the lazy option so that people can disable lazy thumbnails if they don't work nicely for them, virtual scrolling can be laggy (reminder that we use lazy thumbnails to enhance performance during virtual scrolling).

The only reason we need the waitForThumbnailsBeforeUpload option, however, is because some people need exif data.

These seem like unrelated causes, and yet these options are closely intertwined - e.g. using { waitForThumbnailsBeforeUpload: true } automatically disables the lazy option. This doesn't really have to happen, these are compatible!
We can have both performance { lazy: true } and exif data { waitForThumbnailsBeforeUpload: true }.

I think ultimately the correct way to go about these options is to remove the waitForThumbnailsBeforeUpload option, and make "please extract exif data" separate functionality. There are quite some issues about missing exif data from images, people rely on it as if it's intended functionality and not just a side-effect of thumbnail generation.
I'll keep an eye on exif-related issues, let's merge this quick fix for now.

[github-actions]

Diff output files

diff --git a/packages/@uppy/thumbnail-generator/lib/index.js b/packages/@uppy/thumbnail-generator/lib/index.js
index beb1a78..78c7b67 100644
--- a/packages/@uppy/thumbnail-generator/lib/index.js
+++ b/packages/@uppy/thumbnail-generator/lib/index.js
@@ -278,6 +278,7 @@ export default class ThumbnailGenerator extends UIPlugin {
       this.uppy.on("thumbnail:request", this.onFileAdded);
       this.uppy.on("thumbnail:cancel", this.onCancelRequest);
     } else {
+      this.uppy.on("thumbnail:request", this.onFileAdded);
       this.uppy.on("file-added", this.onFileAdded);
       this.uppy.on("restored", this.onRestored);
     }
@@ -292,6 +293,7 @@ export default class ThumbnailGenerator extends UIPlugin {
       this.uppy.off("thumbnail:request", this.onFileAdded);
       this.uppy.off("thumbnail:cancel", this.onCancelRequest);
     } else {
+      this.uppy.off("thumbnail:request", this.onFileAdded);
       this.uppy.off("file-added", this.onFileAdded);
       this.uppy.off("restored", this.onRestored);
     }