When you're looking at a tree view with alternating row colors, and you insert a
row above the visible area, the row colors shift, even though the row data stays
put. This is confusing because it leads the eye to believe the rows are moving
when actually they aren't, and it causes the whole area to be redrawn needlessly.

Fortunately there's a conceptually pretty simple fix for this: Iff the added row
is completely outside and above the visible area, switch the parity of the row
colors for the entire tree, e.g. so that the first row becomes dark instead of
light. This will make the row colors in the visible area appear stable.

I've written a test case that I'm going to attach - it can be added to
gtk+/tests/Makefile.am. It shows a tree view and appends + prepends a row every
1/4 of a second.

I'll start looking into the problem itself now that I have a reliable test.