[Feature request] `gftools flatten-components`

Hi @m4rc1e & @simoncozens

I'm wondering if this maybe a useful sub-tool to have to manipulate binaries.

There are times when binaries in older projects may have nested components which maybe causing issues with printed output, among other things. However, regenerating those fonts to fix the nested components may not be an option because producing new binaries through an improved build process may add regressions. In this case it maybe useful to run something like `gftools flatten-components` to modify the `glyf` table to change the `glyphindex` value to the original, outline glyph.

Could we use the same method as implemented in [fontbakery's 'nested_components'](https://github.com/fonttools/fontbakery/blob/8c5fadc2ed815111e74196d9fa6d669fa545b09c/Lib/fontbakery/checks/nested_components.py) check to modify the `glyf` table?

Thanks in advance!