textview: Avoid relocating adjustments on ::size-allocate while these are animating

textview: Avoid relocating adjustments on ::size-allocate while these are animating

An animation may be scheduled while the textview content changed in size, so the resize
queued would just unset the animation and set the adjusments with a current value,
defeating gtk_text_view_scroll_to_iter(). In this case, just avoid the adjustment change,
as there is a target value on the way.

https://bugzilla.gnome.org/show_bug.cgi?id=733406