fix issue #252, support subgroup aesthetic in geom_polygon for holes

[Nishita-shah1]

Fixes #252

Problem

animint2 rendered isoband hole polygons as two separate filled shapes
instead of one polygon with a transparent hole. The subgroup aesthetic
(added to ggplot2 in 2019) was completely unknown to animint2's compiler
and renderer.

Real-world use case: Hi-C genomic cluster contour visualization where
holes in contour polygons are scientifically meaningful.

After (fix)

Solution

Three-layer fix across the full pipeline:

R/geom-polygon.r - added subgroup = NULL to default_aes so
GeomPolygon accepts the aesthetic.

R/geom-.r -- when subgroup is present in polygon data:

• sets g$data_has_subgroup <- TRUE (serialized into plot.json)
• converts subgroup to character so it survives TSV write
• adds it to g$types so JS type converter keeps it as string

inst/htmljs/animint.js - when data_has_subgroup is true, uses
d3.geo.path().projection(null) with GeoJSON Polygon format instead
of d3.svg.line(). GeoJSON coordinates map directly to subgroup
structure: [outerRing, holeRing1, ...]. SVG fill-rule: evenodd
ensures hole rings appear transparent.

Tests

17 tests, 0 failures covering:

• compiler: subgroup in TSV, data_has_subgroup in plot.json,
  no flag without subgroup
• renderer: SVG path used, multiple M commands in d attribute,
  evenodd fill-rule applied
• interactive: clickID and path checks for all 3 polygon cases
  (hole_and_mid, only_hole, no_hole) as specified in the issue

@tdhock @suhaani-agarwal @Faye-yufan could you please review this, thank you! Fixes #252