GTK 3.20's GtkScrolledWinodw doesn't like having a too small allocation
and spews scary assertion failures. Fix that by requesting the real
size we'd like instead of hard-coding 1 as small-enough value in our
overriding height requisition method.
The actual value doesn't really matter so long as it's small enough
anyway, as we resize the popup to fit later on.
Note: this moves the actual implementation of ListBoxX::GetRowHeight()
to the new convenience function treeViewGetRowHeight(), with no changes
in implementation.
X-Scintilla-Bug-URL: https://sourceforge.net/p/scintilla/bugs/1825/
X-Scintilla-Commit-ID: 5a0afdd87d56d837dd8068e234aed8e2b6bdbe93
abf0365e43