LibWebView: Normalize wheel deltas to CSS pixels

Platform wheel deltas describe movement in the view, not in page CSS
pixels. Convert them by page zoom before dispatch so physical scrolling
covers a smaller page area when zoomed in.

Do this before the UI-side async scrolling shortcut so compositor
scrolling and main-thread fallback scrolling use the same delta. Also
avoid applying the Qt device pixel ratio while converting angle-only
wheel input, since LibWeb later handles device-pixel scaling itself.