(1) Get rid of supports_clip flag. All widgets (implicitly) support
clip.

(2) Don't reset the clip to { 0, 0, 0, 0 } before the "size-allocate"
signal.

(3) Make gtk_widget_set_allocation() set the clip (to the allocation).
This ensures that eveyr widget has a clip set.
Note: It overrides previous calls to gtk_widget_set_clip(), while in
3.14 this didn't happen.

(4) As the clip is set by gtk_widget_set_allocation() now, don't set
it after the "size-allocate" signal anymore.

This fixes calls to gtk_widget_queue_draw() from inside the
size_allocate vfunc.