render exterior ring or interior rings
This option is shown whenever a simple line symbol or
marker line symbol is used as part of a fill symbol for
rendering polygons.
The default behavior is to render both interior and exterior
rings, but this new setting allows users to set the symbol
layer to render only for the exterior ring OR only
for interior rings.
This allows for symbolisation which wasn't directly possible
before, such as a marker line with markers for interior
rings angled toward the interior of the polygon.
Sponsored by the German QGIS User Group
Fixes#12652
