[APV] Fix floating point issue when sampling

When the sampling position was located exactly on the boundary of the last brick of the cell, we would sample the wrong cell due to floating point precision issue.
Fixed by clamping the brick index to the size of the cell.

Author: adrien-de-tocqueville

Packages/com.unity.render-pipelines.core/Runtime/Lighting/ProbeVolume/ProbeVolume.hlsl

@@ -304,6 +304,7 @@ uint GetIndexData(APVResources apvRes, float3 posWS)
         {
             float3 residualPosWS = posWS - topLeftEntryWS;
             int3 localBrickIndex = floor(residualPosWS / (_MinBrickSize * stepSize));
+            localBrickIndex = min(localBrickIndex, (int3)(3 * 3 * 3 - 1)); // due to floating point issue, we may query an invalid brick
 
             // Out of bounds.
             isValidBrick = all(localBrickIndex >= minRelativeIdx) && all(localBrickIndex < maxRelativeIdxPlusOne);