Improve adaptive render resolution for high-DPI displays.

[RZDESIGN]

Cap effective internal pixel ratio by viewport budget and centralize resolution application across init, resize, and quality updates for smoother fullscreen performance.

[RZDESIGN]

https://x.com/RicardoDeZoete/status/2028846395499626544?s=20

[RZDESIGN]

Reviewer checklist for this PR (adaptive Retina resolution):

• Validate fullscreen behavior on high-DPI displays (Retina/4K): confirm reduced GPU load and smoother frame pacing versus main.
• Validate quality impact: ensure expected softness only appears when hitting pixel budget cap; windowed mode should remain sharper when under budget.
• Validate dynamic updates: change resolution multiplier in settings and confirm internal render resolution + post-processing resize update immediately.
• Validate resize path: repeatedly toggle fullscreen/windowed and verify no stale render target sizes, stretching, or post-processing artifacts.
• Regression check: confirm non-fullscreen / low-DPI behavior remains unchanged.

Known intentional trade-off:

• This introduces adaptive capping, so effective pixel ratio may be lower than requested at very large fullscreen resolutions.