Add spotlight custom shader

kitty/shaders/custom/spotlight.pipeline

@@ -0,0 +1,5 @@
+startgroup
+    animation_start os-window-focus-in | user-activity
+    animation_stop os-window-focus-out | user-idle
+    shaders spotlight
+endgroup

kitty/shaders/custom/spotlight.slang

@@ -0,0 +1,63 @@
+#language slang 2026
+// Spotlight effect
+// Original GLSL by Paul Robello, ported to Slang for kitty
+
+import kitty_custom_shader_types;
+
+extern static const float SPOTLIGHT_RADIUS   = 0.25;  // spotlight radius in UV units
+extern static const float SPOTLIGHT_SOFTNESS = 20.0;  // edge sharpness; higher = sharper
+extern static const float AMBIENT_LIGHT      = 0.5;   // minimum brightness outside the spotlight
+extern static const float SPOTLIGHT_SPEED    = 1.0;   // oscillation speed in random-motion mode
+// Set to 1 to follow the mouse, 0 to use random oscillation
+extern static const int   USE_MOUSE          = 1;
+// When USE_MOUSE=1 and the mouse leaves the window:
+//   0 = fall back to random oscillation
+//   1 = clamp spotlight to the nearest window edge
+extern static const int   MOUSE_CLAMP_TO_EDGE = 0;
+
+public float4 fragment_main(
+    float4 color,
+    KittyTextures t,
+    KittyCustomShaderData d,
+    float4 viewport,
+    float animation_progress
+) {
+    float2 uv = t.pos;
+
+    // Viewport pixel dimensions for aspect-ratio correction
+    float2 pixelDims   = float2(d.viewport_size_pixels) * viewport.zw;
+    float  aspectRatio = pixelDims.x / pixelDims.y;
+
+    // Random oscillating center (different frequencies and phases on each axis)
+    float  time         = d.timestamp * SPOTLIGHT_SPEED;
+    float2 randomCenter = float2(
+        0.5 + 0.4 * sin(time),
+        0.5 + 0.4 * sin(time * 1.3 + 3.14159)
+    );
+
+    // Mouse is valid only when both coords are inside the OS window [0, 1]
+    float2 mp            = d.mouse_pos.xy;
+    float  mouseInWindow = float(all(mp >= float2(0.0)) && all(mp <= float2(1.0)));
+
+    // When mouse leaves the window: clamp to nearest edge or fall back to random
+    float2 outsideFallback = lerp(randomCenter, clamp(mp, float2(0.0), float2(1.0)), float(MOUSE_CLAMP_TO_EDGE));
+
+    // Branchless center selection
+    float2 mouseCenter     = lerp(outsideFallback, mp, mouseInWindow);
+    float2 spotlightCenter = lerp(randomCenter, mouseCenter, float(USE_MOUSE));
+
+    // Euclidean distance in aspect-ratio-corrected space (produces a circular spotlight)
+    float distToCenter = length((uv - spotlightCenter) * float2(aspectRatio, 1.0));
+
+    // Soft spotlight edge
+    float spotlightIntensity = smoothstep(
+        SPOTLIGHT_RADIUS,
+        SPOTLIGHT_RADIUS - (1.0 / SPOTLIGHT_SOFTNESS),
+        distToCenter
+    );
+
+    // Blend between ambient and full illumination
+    float3 result = color.rgb * lerp(float3(AMBIENT_LIGHT), float3(1.0), spotlightIntensity);
+
+    return float4(result, color.a);
+}