This renderer merges (or unions/dissolves) the line or polygon features
from a layer prior to rendering them. It's useful for rendering
a polygon layer with overlapping features as one single "coverage" feature,
or a line layer consisting of many smaller component line features
using a regularly spaced marker line or similar.
Internally, this just moves the guts of the existing inverted polygons
renderer to a new base class, as that renderer already had an option
to merge features prior to rendering. Basically it just exposes a new
renderer to users which is the inverted polygon renderer without
the inversion step!
