[photos] Make adjustment filter sliders snap to 0 when moving to neutral point

[sv3nnie]

When adjusting filters (brightness, contrast, etc.), make sliders snap to 0 when the user moves back to the neutral point, to avoid accidentally leaving a tiny offset (e.g., -1 or +1) after releasing. Right now when I try to return a slider to exactly 0, it jumps to -1 or +1 as soon as I release.

Ente.Photos.mov

Apple Photos handles this nicely: it snaps to exactly 0 when you move directly back to the neutral point, but still allows fine adjustments from 0 to small positive or negative values.

Apple.Photos.snap.mov