Root cause of scroll jank: first-present of each Image uploaded pixels
inside the render frame (~11-30 ms per 400px texture, 1.3 visible
hitches/s at 120 Hz; grey-image A/B: 20 -> 1). Slint has no render
thread and no pre-upload API.
Fix: decode worker creates the Rgba8Unorm texture on Slint's own wgpu
device (captured from GraphicsAPI::WGPU30 at RenderingSetup) and submits
the async upload; UI wraps it in Image::try_from(tex). Zero pixel work
in the render frame. CPU from_rgba8 fallback retained.
Measured (Pixel 9 Pro 120 Hz, 400 px, per-frame FR hook):
before: >10ms renders 20 in 15s (max 30 ms), 3 borderline 8-9 ms
after: 0 mid-scroll renders > 4.4 ms (median 1.9, p95 2.9);
only post-idle first-frames expensive (invisible)
Also: FR per-frame logging via set_rendering_notifier; APLY timestamp.
