Zoom plugin displays garbage if running in a nested session

[dkondor]

Describe the bug
If Wayfire is run nested, the zoom plugin does not work properly: images are not updated (or show a weird "shaking" behavior), or only parts are updated. Note: I've tested this so far with the Wayland backend (running on Wayfire); I could not test the X11 backend (gives an error "X11 does not support DRI3 extension").

Note that things seem to return to normal when switching to / from the nested window (I assume this triggers a full repaint?).

To Reproduce
Steps to reproduce the behavior:

1. Run Wayfire nested in a Wayland session
2. Enable zoom plugin
3. Start zoom
4. Screen gets messed up

Expected behavior
Zoom works normally.

Screenshots or stacktrace
Before zooming:

With zoom active, while moving the mouse randomly:

Note: screenshots were taken in the "outer" compositor. If I take a screenshot from the nested Wayfire instance, it is rendered correctly.

Wayfire version
git master (actually, this branch, but there shouldn't be any difference)