make UI text rendering camera driven

[mockersf]

Objective

• Fixes closing the main window change UI in the second window #13687

Solution

• Text rendering in UI is still dependent on the PrimaryWIndow
• implements Camera-driven UI #10559 for text rendering

There are other parts of UI that are still PrimaryWindow dependent, if the changes here are OK I'll apply them everywhere.
I'm not a fan of the EntityHashMap here to hold the scale factors, but it seems the quick and easy fix

Testing

• Run example multiple_windows on a screen with a scale factor different than 1, close the primary window

[rparrett]

Side-question (this PR seems useful regardless): should Bevy promote another Window to PrimaryWindow when the PrimaryWindow is despawned? Docs aren't super clear about what PrimaryWindow means.

[mockersf]

Side-question (this PR seems useful regardless): should Bevy promote another Window to PrimaryWindow when the PrimaryWindow is despawned? Docs aren't super clear about what PrimaryWindow means.

I tried that at first (if there is no primary window, promote the first window to it) because it seemed simpler, but that didn't work nicely. Do you despawn cameras that are targeting the primary window with the primary window? We should have an answer for that, but that need more design...

[rparrett]

tried that at first (if there is no primary window, promote the first window to it) because it seemed simpler, but that didn't work nicely. Do you despawn cameras that are targeting the primary window with the primary window? We should have an answer for that, but that need more design...

Yeah, it feels like something the user should manage to me. I would imagine that most of the time, users would not want to allow the primary window to be despawned in the first place. Just surfacing the idea because it seemed like an "obvious" fix.

[alice-i-cecile]

I'd like to see this done across the UI stack: multi-window UI is the primary consumer of multiple windows.

[mockersf]

Side-question (this PR seems useful regardless): should Bevy promote another Window to PrimaryWindow when the PrimaryWindow is despawned? Docs aren't super clear about what PrimaryWindow means.

let's discuss in #13698