canvg repends on rgbcolor.js when using fill-opacity

We will probably also want to add the StackBlur.js dependency,
but this can wait until the blur feature is required.

[#57574726]
Render invisible atoms in canvg with fill-opacity: 0

Ignore element and color in key for invisible atoms, so
that invisible atoms of different colors can share the
texture, but keep radius as this will affect mouse hit target.

[#57574726]