Support window independent UI scaling

[Weibye]

What problem does this solve or what need does it fill?

There are now multiple occurrences where the code assumes only one window and proceeds calculations using the primary-window scale factor. This means setting the scale factor of the primary window will incorrectly affect all windows.

Code to be found here:

• bevy_ui
  • widget/text
    • https://github.com/bevyengine/bevy/blob/main/crates/bevy_ui/src/widget/text.rs#L55
  • flex
    • https://github.com/bevyengine/bevy/blob/main/crates/bevy_ui/src/flex/mod.rs#L216
  • render
    • https://github.com/bevyengine/bevy/blob/main/crates/bevy_ui/src/render/mod.rs#L285
• bevy_text/text2d
  • https://github.com/bevyengine/bevy/blob/main/crates/bevy_text/src/text2d.rs#L84
  • https://github.com/bevyengine/bevy/blob/main/crates/bevy_text/src/text2d.rs#L169

What solution would you like?

These systems should instead be fetching the actual scale of the current window the UI tree is a part of instead of the PrimaryWindow.

What alternative(s) have you considered?

• Might require Support multiple UI roots #5622
• Might require Viewport Specific UI #5570
• Or a variation of that: "Window-specific UI"