feat: add spot light shadows with a tilled buffer

destrum/assets_src/shaders/lights.glsl

@@ -7,6 +7,10 @@
 #define TYPE_POINT_LIGHT 1
 #define TYPE_SPOT_LIGHT 2
 
+#define NO_SHADOW 0xFFFFFFFFu
+// Must match ShadowMapPipeline::MAX_SPOT_SHADOWS
+#define MAX_SPOT_SHADOWS 16
+
 // Forward+ tiling, must match LightCullingPipeline::TILE_SIZE / MAX_LIGHTS_PER_TILE
 #define LIGHT_TILE_SIZE 16
 #define MAX_LIGHTS_PER_TILE 255
@@ -27,10 +31,17 @@ struct Light {
 
     float spotInnerCos;
     float spotOuterCos;
-    uint castShadows;
+    // NO_SHADOW, or the light's slot in the shadow data of its type
+    uint shadowIndex;
     float padding;
 };
 
+// Must match GPUSpotShadow in Light.h
+struct SpotShadow {
+    mat4 lightViewProj;
+    float normalOffsetPerMeter;
+};
+
 layout (buffer_reference, std430) readonly buffer LightsBuffer {
     Light data[];
 };