Improve label layout when horizontally flipped

[cmdcolin]

After floating labels merge #2781 , the horizontally flipped mode can have some label collisions

To properly do the layout, the layout should basically be done in the reverse orientation (if that makes sense...)

Basically the labels should accommodate feature label overflow space to the right of the feature when horizontally flipped, but currently it is laid out in the forward orientation, and then that space is allocated to the left of the feature when it is horizontally flipped

We merged this before it's pixel perfect since it is quite helpful to have some floating labels

[cmdcolin]

labelled as high impact since synteny views often require this