SVG font support

[nulano]

FreeType 2.12 (#6169) adds support for color fonts in SVG format.

From https://github.com/freetype/freetype/commit/21d0fa374200aecc6605daf034a47167ea82ed6f:

FreeType now handles OT-SVG fonts, to be controlled with FT_CONFIG_OPTION_SVG configuration macro. By default, it can only load the 'SVG ' table of an OpenType font. However, by using the svg-hooks property of the new 'ot-svg' module it is possible to register an external SVG rendering engine. The FreeType demo programs have been set up to use 'librsvg' as the rendering library.

It would be nice to add support for this to Pillow. librsvg appears to be LGPL-2.1 licensed, so a wrapper similar to that used for FriBiDi/Raqm may be needed. This is closely related to loading SVG images in Pillow, it may be possible to add support for both at once: #3509

For reference, the freetype demo callbacks are here: https://gitlab.freedesktop.org/freetype/freetype-demos/-/blob/master/src/rsvg-port.c