Tooltip: Fix the used cursor size if 0 in Settings
Before the recent rework of positioning in GtkTooltip, the widget always used the cursor_size of the GdkDisplay. That work redid this to instead take GtkSettings::gtk-cursor-theme-size. But that property's doc says:
Size to use for cursors, or 0 to use the default size.
and has 0 as its default. This is quite a likely scenario for anyone whose desktop or settings.ini does not explicitly provide a cursor size, which is the case for XFCE and win32, to name just two common platforms.
Then, it seems getting a cursor_size of 0 causes GtkTooltip to freak out and hide/show itself at a very rapid speed, thus making it unusable.
So, we should check whether the Settings return 0 and, if so, still use gdk_display_get_default_cursor_size (display) to ensure we get a size.