Array: add option to centre grid on the original location

Especially for SMD footprint array, it's awkward to have to
offset the pad to the corner first before making the array.

Relates-To: https://gitlab.com/kicad/code/kicad/-/issues/16882

common/array_options.cpp

@@ -49,7 +49,7 @@ VECTOR2I ARRAY_GRID_OPTIONS::getGridCoords( int n ) const
 }
 
 
-ARRAY_OPTIONS::TRANSFORM ARRAY_GRID_OPTIONS::GetTransform( int n, const VECTOR2I& aPos ) const
+VECTOR2I ARRAY_GRID_OPTIONS::gtItemPosRelativeToItem0( int n ) const
 {
     VECTOR2I point;
 
@@ -74,7 +74,25 @@ ARRAY_OPTIONS::TRANSFORM ARRAY_GRID_OPTIONS::GetTransform( int n, const VECTOR2I
         point += stagger_delta * copysign( stagger_idx, m_stagger ) / stagger;
     }
 
-    // this is already relative to the first array entry
+    return point;
+}
+
+
+ARRAY_OPTIONS::TRANSFORM ARRAY_GRID_OPTIONS::GetTransform( int n, const VECTOR2I& aPos ) const
+{
+    VECTOR2I point = gtItemPosRelativeToItem0( n );
+
+    // Bump the item by half the array size
+    if( m_centred )
+    {
+        // Get the array extents
+        const int arrayExtentX = ( m_nx - 1 ) * m_delta.x + ( m_ny - 1 ) * m_offset.x;
+        const int arrayExtentY = ( m_ny - 1 ) * m_delta.y + ( m_nx - 1 ) * m_offset.y;
+
+        std::cout << "Subtracting " << VECTOR2I( arrayExtentX, arrayExtentY ) / 2 << std::endl;
+        point -= VECTOR2I( arrayExtentX, arrayExtentY ) / 2;
+    }
+
     return { point, ANGLE_0 };
 }