scroll: replace dx + dy with dominant axis, correct sign

[kofa73]

Scrolling logic used to calculate delta = dx + dy. Two issues:

• usually, we want up and right to mean the same thing (increase a value, a diameter), but up = dy < 0, right = dx > 0, so right was treated as equivalent to down
• diagonals would either amplify or reduce the effect (e.g. moving up (dy < 0) + right (dx > 0) on a touchpad could potentially cancel out)

Consistency fixes:

• Zoom up/right = in, down/left = out:

  • Darkroom center image zoom (discrete/non-touchpad path)
  • Darkroom 2nd window zoom
  • Color zones zoom (hold a)
  • Slider pop-up zoom (resolution/range)
  • Timeline zoom (Ctrl+scroll)

• Value: up/right = increase / down/left = decrease

  • Sliders — value up/down and the reversed left/right tilt
  • Color correction — saturation
  • Tone equalizer — brighten/darken on hover
  • Graduated density — density (Ctrl) and hardness (Shift)
  • Liquify — stamp size, effect strength, and rotate direction
  • Zone system — number of zones

• effect circle: up/right = enlarge, down/left = shrink

  • Contrast equalizer (atrous), denoise (profiled), raw denoise, lowlight, color zones (edit-by-area) -- feedback circles
  • Monochrome — selection circle radius
  • Rotate & perspective — analysis selection radius

• Mask / shape scale: up/right = increase, down/left = decrease

  • Drawn mask overlays — circle, ellipse, brush, path, gradient, object, group

• Node / handle / marker move: up/right = move up; down/left: down

  • Base curve / tone curve / RGB curves — point up/down
  • Color equalizer — graph nodes up/down
  • RGB levels / levels — histogram handles
  • Retouch — wavelet/decomposition bar (top slider, bottom slider, dot position)
  • Gradient slider active marker (e.g. retouch wavelet-scale preview handle)

partial fix for issues listed in #20843 (plus a few others identified later)