PR summary

This adds support for fonts with colour glyphs supported by FreeType. Specifically, this should mean COLRv0 fonts. There also exist some other colour font types, which arenot supported:

• CBDT is a non-scalable bitmap format and we don't support those, but it may be possible if we do a scaling ourselves.
• SVG requires a parser (though it's some font-specific subset of the whole SVG spec)
• COLRv1 essentially requires a full renderer setup.

We of course do have a full renderer available, but that would require much more interfacing to get working. HarfBuzz (which we use indirectly through libraqm) also has some API for these COLRv1 fonts, but I don't know if it's any nicer to use than FreeType's. Unfortunately, this does exclude one of the most popular emoji fonts, Noto Color Emoji, as that has moved to COLRv1+SVG.

This PR is based on all open font work, because a)#30059 makes it much easier to place the colour data if we don't have to use an intermediate buffer, and b)#30607 due toFT_Glyph_To_Bitmap losing colour information and so we need to move to an implementation that usesFT_Render_Glyph directly. I also merged#30334 though it's probably not strictly required.

For example, we can now render Niklaas in COLRv0, some fonts with simpler decorative effects like Cairo Play, and the older OpenMoji Color that was COLRv0. Fonts that use SVG (like Nabla and Gilbert here) are reduced to their greyscale variant.

PR checklist

• "closes #0000" is in the body of the PR description tolink the related issue
• new and changed code istested
• Plotting related features are demonstrated in anexample
• New Features andAPI Changes are noted with adirective and release note
• Documentation complies withgeneral anddocstring guidelines