Include interactive brushes in PNG/PDF export files

[sehilyi]

Since brushes are rendered in SVG elements, they are not included in PNG files along with WebGL graphics. We need to change SVG elements to WebGL graphics or dynamically render brushes upon exporting PNG files.

Screenshot of Actual Visualization

Exported PNG File

[sehilyi]

This can be done by setting the opacity of the current SVG-based brush elements 0 and drawing the identical Arc graphics using Pixi.js, i.e., the current SVG elements are for interaction, and the Pixi.js elements are for display.