Can be useful when using HarfBuzz for font loading and shaping
but using DirectWrite for rendering.

https://github.com/harfbuzz/harfbuzz/issues/1641

It makes a hb_face_t from IDWriteFontFace, useful when using
DirectWrite facilities for font selection, loading and rendering
but using harfbuzz for shaping.

Fixes #1548 hopefully

hb-directwrite.cc: In constructor 'TextAnalysis::TextAnalysis(const wchar_t*, uint32_t, const wchar_t*, DWRITE_READING_DIRECTION)':
hb-directwrite.cc:489:18: error: 'TextAnalysis::mText' will be initialized after [-Werror=reorder]
   const wchar_t* mText;
                  ^~~~~
hb-directwrite.cc:488:12: error:   'uint32_t TextAnalysis::mTextLength' [-Werror=reorder]
   uint32_t mTextLength;
            ^~~~~~~~~~~
hb-directwrite.cc:282:3: error:   when initialized here [-Werror=reorder]
   TextAnalysis (const wchar_t* text, uint32_t textLength,
   ^~~~~~~~~~~~
hb-directwrite.cc: In function 'hb_bool_t _hb_directwrite_shape_full(hb_shape_plan_t*, hb_font_t*, hb_buffer_t*, const hb_feature_t*, unsigned int, float)':

No other shaper will need shape_plan_data, by definition.  So, remove
abstraction layer and always create hb_ot_shape_plan_t as part of
hb_shape_plan_t.

Sorry for the noise, downstream custom builders.  Please adjust.

Fixes #791.

Fixes https://github.com/behdad/harfbuzz/issues/615

We'll see which bots I broke.

Part of fixing https://github.com/behdad/harfbuzz/pull/605

Shape-plan caching is not implemented.