popover: Clamp tail gap limits by the tail height on that side

popover: Clamp tail gap limits by the tail height on that side

All popover sides have extra margins to possibly hold the tail, this is
accounted for in gtk_popover_get_rect_coords(), and should be accounted
for too in the tail position calculation.

This fixes the gtk_render_frame_gap() warnings seen when a popover is
pushed far too close to window sides.