SVG document corresponding to a glyphID is fetched and stored
in an `FT_SVG_Document' structure which is referenced in the
glyphslot.
* include/freetype/config/ftheader.h: Adds `FT_OTSVG_H'.
* include/freetype/ftimage.h: Adds `FT_GLYPH_FORMAT_SVG'.
* include/freetype/internal/ftobjs.h: Adds `FT_GLYPH_OWN_GZIP_SVG'.
* include/freetype/internal/sfnt.h: Adds `load_svg_doc' and its
function type `TT_Load_Svg_Doc_Func'.
* include/freetype/otsvg.h: Adds `FT_SVG_Document' and its struct.
* src/base/ftobjs.c: Adds code to allocate and free memory for
`FT_SVG_Document' in `slot->other'.
* src/cff/cffgload.c: Adds code to load SVG glyph if it's present.
* src/truetype/ttgload.c: Ditto.
* src/sfnt/sfdriver.c: Adds `tt_face_load_svg_doc'.
* src/sfnt/ttsvg.h: Ditto.
* src/sfnt/ttsvg.c: Adds implementation of `tt_face_load_svg_doc'
and its helper functions.