优化体积渲染：使用 HDDA 树遍历器

改进内容：
1. 添加 get_hdda_hit 函数，使用 pnanovdb_hdda_tree_marcher
2. 添加 get_dim_coord 函数，检测空区域维度
3. 优化 ray marching：
   - 先用 HDDA 找到第一个有数据的点
   - 遍历中检测 dim，跳过空区域
   - 回溯机制避免跳过有效数据

效果：体积云渲染更完整，不再是只有框表面有数据

Res/Shader/volume.hlsl

@@ -59,6 +59,31 @@ float get_value_coord(inout pnanovdb_readaccessor_t acc, float3 pos)
     return pnanovdb_read_float(buf, address);
 }
 
+uint get_dim_coord(inout pnanovdb_readaccessor_t acc, float3 pos)
+{
+    pnanovdb_vec3_t p = pos;
+    pnanovdb_coord_t ijk = pnanovdb_hdda_pos_to_ijk(p);
+    return pnanovdb_readaccessor_get_dim(PNANOVDB_GRID_TYPE_FLOAT, buf, acc, ijk);
+}
+
+bool get_hdda_hit(inout pnanovdb_readaccessor_t acc, inout float tmin, float3 origin, float3 direction, float tmax, out float valueAtHit)
+{
+    pnanovdb_vec3_t p_origin = origin;
+    pnanovdb_vec3_t p_direction = direction;
+    float thit;
+    bool hit = pnanovdb_hdda_tree_marcher(
+        PNANOVDB_GRID_TYPE_FLOAT,
+        buf,
+        acc,
+        p_origin, tmin,
+        p_direction, tmax,
+        thit,
+        valueAtHit
+    );
+    tmin = thit;
+    return hit;
+}
+
 PSInput MainVS(VSInput input)
 {
     PSInput output;
@@ -100,19 +125,45 @@ float4 MainPS(PSInput input) : SV_TARGET
     float3 color = float3(0, 0, 0);
     float transmittance = 1.0;
     
+    float not_used;
+    bool hit = get_hdda_hit(volume.acc, tmin, _CameraPos, rayDir, tmax, not_used);
+    if (!hit) { return float4(0, 0, 0, 0); }
+    
+    float skip = 0;
+    
     for (uint i = 0; i < _MaxSteps; i++) {
-        float t = tmin + i * _StepSize;
-        if (t > tmax || transmittance < 0.01) break;
+        if (tmin >= tmax || transmittance < 0.01) break;
         
-        float3 worldPos = _CameraPos + rayDir * t;
+        float3 worldPos = _CameraPos + rayDir * tmin;
+        
+        uint dim = get_dim_coord(volume.acc, worldPos);
+        if (dim > 1) {
+            float skip_step = 15.0;
+            tmin += skip_step;
+            skip = skip_step;
+            continue;
+        }
         
         float density = get_value_coord(volume.acc, worldPos) * _DensityScale;
         
-        if (density > 0.001) {
-            float3 S = density * float3(1, 1, 1);
-            color += transmittance * S * _StepSize;
-            transmittance *= exp(-density * _StepSize);
+        if (density < 0.001) {
+            float skip_step = 5.0;
+            tmin += skip_step;
+            skip = skip_step;
+            continue;
         }
+        
+        if (skip > 0) {
+            tmin -= skip * 0.8;
+            worldPos = _CameraPos + rayDir * tmin;
+            skip = 0;
+        }
+        
+        float3 S = density * float3(1, 1, 1);
+        color += transmittance * S * _StepSize;
+        transmittance *= exp(-density * _StepSize);
+        
+        tmin += _StepSize;
     }
     
     float alpha = 1.0 - transmittance;