popover: Correctly substract arrow height when measuring popover widget

popover: Correctly substract arrow height when measuring popover widget

For default popover arrow position and default height-for-width layout mode,
natural_width is calculated first with for_size=-1 and orientation=HORIZONTAL,
at the end of gtk_popover_measure() natural_width  won't be added with tail_height.

Then to measure with for_size=natural_width and orientation=VERTICAL, obviously
for_size shouldn't be substract with tail_height.

The wrong logic will force content in popover gets less width and then text labels
in popover may get wrapped unnecessarily.