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/Common/Theme.qml

@@ -2015,6 +2015,23 @@ Singleton {
         }
     }
 
+    // Returns numeric fillMode value for shader use (matches shader calculateUV logic)
+    function getShaderFillMode(modeName) {
+        switch (modeName) {
+        case "Stretch": return 0;
+        case "Fit":
+        case "PreserveAspectFit": return 1;
+        case "Fill":
+        case "PreserveAspectCrop": return 2;
+        case "Tile": return 3;
+        case "TileVertically": return 4;
+        case "TileHorizontally": return 5;
+        case "Pad": return 6;
+        case "Scrolling": return 7;
+        default: return 2;
+        }
+    }
+
     function snap(value, dpr) {
         const s = dpr || 1;
         return Math.round(value * s) / s;