Setting background colours to a QTableWidgetItem that is used as a horizontal header item in QTableWidget doesn't display correctly in 'fusion' style.

I believe this is due to the `QPalette::cacheKey()` being equal for palette with different background brushes, as in QFusionStyle::drawControl under case CE_HeaderSection: reuses the same cached pixmap for at least some of the header items.

Can be reproduced using the following code added to WidgetGallery::createBottomLeftTabWidget() in the 'styles' example: 

tableWidget->setColumnCount(4);
const auto headerColours = std::vector<QColor>{Qt::GlobalColor::red, Qt::GlobalColor::blue, Qt::GlobalColor::green};
for (size_t i=0; i<headerColours.size(); ++i)
{
  const auto &colour = headerColours[i];
  auto headerItem = new QTableWidgetItem("");
  headerItem->setBackground(colour);
  tableWidget->setHorizontalHeaderItem(i, headerItem);
}

When displaying in 'fusion mode' the headers are coloured incorrectly in Qt 6:

In Qt 5 they are displayed correctly: