shader-based scrolling wallpaper mode (#1802)

* feat: parallax-scroll wallpaper

Add a `Scrolling` wallpaper fill mode that translates the wallpaper crop
with the active workspace — like Android home-screen parallax, but along
niri's vertical workspace axis.

The image is scaled to cover the screen on its non-scroll axis, and the
active workspace index drives a fractional offset into the cropped
overflow along the scroll axis. Scroll position is spring-animated
CPU-side and handed to a minimal single-texture shader as a UV offset.
Per-monitor scroll position is published into SessionData so the lock
screen renders the same crop as the active workspace, keeping visual
continuity across lock/unlock.

Two implementation details worth calling out for review:

- QSG_USE_SIMPLE_ANIMATION_DRIVER=1 is exported to the spawned quickshell
  process. The default animation driver advances in fixed ~16ms steps,
  capping the scroll at 60Hz and desyncing it from compositor motion on
  high-refresh displays; the simple driver advances by real elapsed time,
  restoring native-refresh pacing. Removing it visibly regresses to 60Hz.

- The wallpaper survives wl_output rebind cycles (e.g. OLED image-cleaning
  on DPMS soft-off), which otherwise leave a stuck or void background.
  Recovery re-anchors the scroll target on output-lifecycle signals,
  rebuilds the ShaderEffect against the current render context, and
  re-attaches the wallpaper-layer surface on unlock for parallax-active
  monitors — guarded against lock state so the shader gets reliable frame
  hints.

* simplify bindings and gate lock screen shader in a loader

---------

Co-authored-by: bbedward <bbedward@gmail.com>

quickshell/Modules/Settings/WallpaperTab.qml

@@ -307,9 +307,9 @@ Item {
 
                     DankButtonGroup {
                         id: fillModeGroup
-                        property var internalModes: ["Stretch", "Fit", "Fill", "Tile", "TileVertically", "TileHorizontally", "Pad"]
+                        property var internalModes: ["Stretch", "Fit", "Fill", "Scrolling", "Tile", "TileVertically", "TileHorizontally", "Pad"]
                         anchors.horizontalCenter: parent.horizontalCenter
-                        model: [I18n.tr("Stretch", "wallpaper fill mode"), I18n.tr("Fit", "wallpaper fill mode"), I18n.tr("Fill", "wallpaper fill mode"), I18n.tr("Tile", "wallpaper fill mode"), I18n.tr("Tile V", "wallpaper fill mode"), I18n.tr("Tile H", "wallpaper fill mode"), I18n.tr("Pad", "wallpaper fill mode")]
+                        model: [I18n.tr("Stretch", "wallpaper fill mode"), I18n.tr("Fit", "wallpaper fill mode"), I18n.tr("Fill", "wallpaper fill mode"), I18n.tr("Scroll", "wallpaper fill mode"), I18n.tr("Tile", "wallpaper fill mode"), I18n.tr("Tile V", "wallpaper fill mode"), I18n.tr("Tile H", "wallpaper fill mode"), I18n.tr("Pad", "wallpaper fill mode")]
                         selectionMode: "single"
                         buttonHeight: 28
                         minButtonWidth: 48