Unwrap multiple layers of themed resources

[dweymouth]

Description:

This PR adds a new unwrapResource function to retrieve the "base" resource from a nested chain of themed resource types, and calls it in the Name and Content functions of all ThemedResource types to achieve two improvements:

• Avoid multiple calls to svg.Colorize, that allocate excess memory and waste CPU
• Avoid caching multiple versions of the same SVG rasterization with differing prefix names like "error_foreground_base-resource.svg" and "error_base-resource.svg".

Fixes #(issue)

Checklist:

• Tests included.
• Lint and formatter run with no errors.
• Tests all pass.

[andydotxyz]

Thanks