v0.7.72

• New callout terminator: callout-end=ring, a hollow circle around the anchor.
• Added inner and outer glows to polygons: new -style options outer-glow-color=, outer-glow-width=, outer-glow-opacity=, inner-glow-color=, inner-glow-width= and inner-glow-opacity=. Glows are rendered as SVG filters on export.
• [web] Added an Effects section with glow controls to the polygon style panel.
• [web] Emptying a color field in a style panel now removes the color (or restores the default, for colors that can't be removed).