Starting with 77244cd the popup
open-state is cleaned up per memory pass.

This becomes problematic for implementations that share memory between
viewports (i.e. all of them, as far as i understand it), because each
viewport gets a context pass, and thus a memory pass, which cleans out
popup open state.

To illustrate my issue, i have modifed the multiple viewport example to
include a popup menu for the labels:
https://gist.github.com/mkalte666/4ecd6b658003df4c6d532ae2060c7595
(changes not included in this pr).

Then, when i try to use the popups, i get this: 


https://github.com/user-attachments/assets/7d04b872-5396-4823-bf30-824122925528

Immediate viewports just break popup handling in general, while deferred
viewports kinda work, or dont. In this example ill be honest, it kind of
still did, sometimes. In my more complex app with multiple viewports
(where i encountered this first) it also just broke - even when just
showing root and one nother. Probably to do with the order wgpu vs glow
draws the viewports? Im not sure. In any case:

This commit adds `Memory::popup` (now `Memory::popups`) to the
per-viewport state of memory, including viewport aware cleanup, and
adding it to the list of things cleaned if a viewport goes away.

This results in the expected behavior:



https://github.com/user-attachments/assets/fd1f74b7-d3b2-4edc-8dc4-2ad3cfa2160e

I will note that with this, changing focus does not cause a popup to be
closed, which is consistent with current behavior on a single app.

Hope this helps 

~Malte 

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