A windowed render dehazes with the whole frame's airlight, estimated on a 384 px proxy of the frame
(`local::frame_airlight`). Every window asked for that proxy again, so every pan of a 1:1 view and every
slider step while zoomed sampled the whole original down to it on the CPU: ~200 ms of CPU on a 64 MP raw.
The proxy (taken before white balance) is now kept for the last 4 (source fingerprint, frame, size); white
balance, the dark channel and the percentile still run on it every time. Same pixels: the GPU window of
`render_bench` is byte-identical.
`optics::fingerprint` indexed the first pixel of an empty image; it now hashes only the dimensions then.
render_bench ONLY=window GPU_ONLY=1 on a 64 MP Sony ILCE-7RM6 raw (RTX 4070 Ti SUPER, 4-CPU quota), CPU time:
pan 208-212 -> 9-11 ms, exposure change 200-204 -> 4 ms, NR change 211-217 -> 13 ms.
Co-Authored-By: Claude <noreply@anthropic.com>
