treeview: Prevent fuzzy centered expander arrows

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.)
6 jobs for 1464-tree-view-expander-arrows-no-longer-positioned-vertically-centered-correctly in 12 minutes and 23 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #591959
fedora-autotools

00:12:23

manual #591960
allowed to fail manual
fedora-distcheck
passed #591961
fedora-meson

00:09:08

passed #591962
fedora-meson-staticlibs

00:09:32

manual #591964
win32 allowed to fail manual
msys2-mingw32-autotools
passed #591963
win32
msys2-mingw32-meson

00:03:41