Merge remote-tracking branch 'gl/dgobbi/stencil-iterator-overrun-6.3' into release-6.3

* gl/dgobbi/stencil-iterator-overrun-6.3:
  Fix a memory overrun in vtkImageStencilIterator.

Imaging/Core/vtkImageStencilIterator.txx

@@ -114,7 +114,7 @@ void vtkImageStencilIterator<DType>::Initialize(
   if (stencil)
     {
     this->HasStencil = true;
-    this->InStencil = true;
+    this->InStencil = false;
 
     this->SpanIndexX = 0;
     this->SpanIndexY = 0;
@@ -155,11 +155,15 @@ void vtkImageStencilIterator<DType>::Initialize(
     if (yOffset >= 0)
       {
       this->SpanMinY = 0;
+      // starting partway into the stencil, so add an offset
       startOffset += yOffset;
       }
     else
       {
       this->SpanMinY = -yOffset;
+      // starting before start of stencil: subtract the increment that
+      // will be added in NextSpan() upon entry into stencil extent
+      startOffset -= 1;
       }
 
     if (stencilExtent[3] > extent[3])
@@ -175,11 +179,18 @@ void vtkImageStencilIterator<DType>::Initialize(
     if (zOffset >= 0)
       {
       this->SpanMinZ = 0;
+      // starting partway into the stencil, so add an offset
       startOffset += zOffset*this->SpanSliceIncrement;
       }
     else
       {
       this->SpanMinZ = -zOffset;
+      // starting before start of stencil: subtract the increment that
+      // will be added in NextSpan() upon entry into stencil extent
+      if (yOffset >= 0)
+        {
+        startOffset -= 1 + this->SpanSliceEndIncrement;
+        }
       }
 
     if (stencilExtent[5] > extent[5])
@@ -202,14 +213,18 @@ void vtkImageStencilIterator<DType>::Initialize(
         vtkImageStencilIteratorFriendship::GetExtentLists(stencil) +
         startOffset;
 
-      // Holds the current position within the span list for the current row
-      this->SetSpanState(this->SpanMinX);
+      // Get the current position within the span list for the current row
+      if (yOffset >= 0 && zOffset >= 0)
+        {
+        // If starting within stencil extent, check stencil immediately
+        this->InStencil = true;
+        this->SetSpanState(this->SpanMinX);
+        }
       }
     else
       {
       this->SpanCountPointer = 0;
       this->SpanListPointer = 0;
-      this->InStencil = false;
       }
     }
   else