[sfnt] Read WOFF 2 header.

Check for WOFF2 tag, call `woff2_open_font', and implement it to read
header according to specification.

* include/freetype/internal/fttrace.h: Add `sfwoff2.c'.

* src/sfnt/rules.mk (SFNT_DRV_SRC): Add `sfwoff2.c'.

* src/sfnt/sfnt.c: Include `sfwoff2.c'.

* src/sfnt/sfobjs.c (sfnt_open_font): Check for `wOF2' tag and call
`woff2_open_font'.

* src/sfnt/sfwoff2.c, src/sfnt/sfwoff2.h: New files.
7 files changed