We want expander arrows to be vertically centered in their row, so we
pass the cell area's height to the renderer.

However, if the cell area's height is an odd number while the
"expander-size" style property is an even number, or vice versa, the
arrow will be centered in a half pixel, and fuzzily rendered.

So, round the render height to the same parity as the expander-size.

(This is not necessary for the arrow width because it's assumed equal
to the "expander-size" style-property.)