Fixing bugs introduced with v1.16.14

Fixes #477 and #476.
Also fixes an error which prevented an empty interior for 'Polyline' and 'Polygon' annotations, when the stroke color was set.
In addition, the interior of line end symbols (where applicable) is now either white, or the interior of the annotation, or an independent fill color set by Annot.update parameter fill_color. Line ends without an interior area remain unchanged (e.g. open arrows, butt, etc.).