Touchpad: allow simultaneous smartphone-like pinch zoom and pan gestures in culling layouts#20888
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Pinch-zoom and ctrl+scroll in culling re-runs the full mipmap fetch + color transform on every event, so the gesture stutters even though the underlying image content does not change. Cache the native-resolution color-converted mipmap surface per thumbnail and add a deferred preview path: during a gesture _thumb_draw_image rescales the existing surface via a cairo transform, and the proper surface reload is triggered once the gesture finishes via dt_culling_zoom_end(). Add a smooth-scroll ctrl path so touchpad zoom feels seamless rather than batched into 0.5-unit steps. The bound formulas in _zoom_to_center, _event_motion_notify, dt_culling_pan_move and dt_thumbnail_image_refresh_position now derive from thumb->zoom directly so they stay correct while the surface still shows the gesture-start zoom.
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This addresses request (e) of issue #20750.
This change allows smartphone-like simultaneous gesture based pinch zooming and panning within the lighttable culling layouts when having two pictures next to each other.
The behavior works as implemented for the darkroom in #20812.
Screencast.from.2026-04-27.16-26-01.mp4
I start this as a draft PR, because it also contains caching code for the culling layout, which made this work much easier and smoother. I'll try to factor out this code into a separate PR.
Disclaimer: this work was co-created with Claude.