Structural changes: All renderer tasks are top-level classes.
Draft point/line/rect/shape specific aggregation code.

Won't actually work yet because point isn't a java.awt.Shape, but its
getting closer.
A few (draft) spreaders
Square spread tests passing.
Merge branch 'master' into spread
Added unit tests for other spreads.  All passing.
Points and rectangles work.  Shape and line are still untested.
Updated extensions for geometry-aware aggregation.  Updated test suite.  Most tests passing.
Passes all tests.
Merge branch 'spread' into geometricAggregators
Fixed a few testing issues.  Added rectangle-vs-points preset.
Better resource management in tests.
Threadpool can be shared between renderers.
Task subsetting bypasses 'make'
Runtime geometry type detection is now optional.