A slider drag rendered the loupe's whole frame at 0.6 of its size, so the
fit view went soft while dragging. On the GPU a canvas-sized frame takes a
few ms, so drags now render at full size there; the CPU keeps the 0.6 draft
(region::drag_scale, CPU_DRAFT_SCALE). The no-window path's canvas cap now
follows an explicit `drafting` flag instead of `scale < 1.0`, so a GPU drag
at 1:1 still never renders every pixel of the photo each frame.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
