Instead of triming only buffer ranges, add a trim_from method to
display line to keep the initial N columns, we know how many columns
are used by non-trimable widgets in DisplaySetup::widget_columns so
we can just pass this.
Also restore the previous logic for face merging
Fixes#4670
Make the column highlighter faces final, and change final logic to
give precedence to the base face when both the base and new face are
final.
Fixes#4669