Reduce the number of font loading implementations used by this crate

[notgull]

Excerpt of cargo bloat when run on an example programming containing winit and piet-hardware, which uses cosmic-text for text:

 File  .text    Size                  Crate Name
 0.6%   2.1% 77.7KiB                 x11_dl x11_dl::xlib::Xlib::open::{{closure}}
 0.2%   0.7% 27.8KiB                  swash swash::scale::bitmap::resize
 0.2%   0.6% 23.6KiB                  swash swash::scale::glyf::hint::Hinter::execute
 0.2%   0.6% 22.3KiB                    std addr2line::ResDwarf<R>::parse
 0.2%   0.6% 22.3KiB                  winit winit::platform_impl::platform::wayland::event_loop::EventLoop<T>::run
 0.2%   0.5% 20.1KiB                   glow glow::gl46::struct_commands::GlFns::load_all_with_dyn
 0.2%   0.5% 19.9KiB                    std std::backtrace_rs::symbolize::gimli::resolve::{{closure}}
 0.1%   0.5% 17.4KiB                  winit winit::platform_impl::platform::x11::event_processor::EventProcessor<T>::process_event
 0.1%   0.4% 14.8KiB            cosmic_text cosmic_text::shape::ShapeLine::layout
 0.1%   0.4% 14.6KiB                   piet piet::samples::samples_main
 0.1%   0.4% 14.2KiB                    png png::decoder::stream::StreamingDecoder::update
 0.1%   0.3% 13.0KiB            wayland_sys wayland_sys::client::WaylandClient::open
 0.1%   0.3% 13.0KiB             ttf_parser ttf_parser::tables::cff::cff1::_parse_char_string
 0.1%   0.3% 13.0KiB             ttf_parser ttf_parser::tables::cff::cff1::_parse_char_string
 0.1%   0.3% 12.6KiB           glutin_winit glutin_winit::finalize_window
 0.1%   0.3% 12.2KiB                  winit winit::platform_impl::platform::x11::window::UnownedWindow::new
 0.1%   0.3% 11.5KiB             ttf_parser ttf_parser::tables::cff::cff2::_parse_char_string
 0.1%   0.3% 11.5KiB             ttf_parser ttf_parser::tables::cff::cff2::_parse_char_string
 0.1%   0.3% 11.1KiB                  swash swash::scale::cff::cff::Glyph::parse_imp
 0.1%   0.3% 10.8KiB smithay_client_toolkit smithay_client_toolkit::seat::keyboard::ffi::XkbCommon::open

The CFF font parsing tables from ttf_parser add about 50 KiB to the binary while the one that swash uses adds about 12 KiB to the binary. It would be nice if only one instance of the CFF table was present in the final binary.

cc dfrg/swash#36