We do this by using `totalSfntSize' as an initial reference, and
extending the buffer when required. This reduces rendering time
considerably.
* include/freetype/internal/wofftypes.h (WOFF2_HeaderRec): Add
`totalSfntSize', rename `total_sfnt_size' to `actual_sfnt_size'.
* src/sfnt/sfwoff2.c (write_buf): Add parameter `dst_size' to keep
track of and update total size of stream.
(WRITE_SFNT_BUF, WRITE_SFNT_BUF_AT): Modify macros accordingly.
(pad4, store_loca, reconstruct_glyf, reconstruct_hmtx,
reconstruct_font): Update parameters to accept `sfnt_size'.
(woff2_open_font): Add variable `sfnt_size'. Use WOFF2 header field
`totalSfntSize' as initial reference (if value makes sense) and
allocate `totalSfntSize' bytes for the sfnt stream. `write_buf'
handles reallocation if and when required. Also resize the stream
to `actual_sfnt_size' after reconstruction.
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