avcodec/webvttenc: add webvtt encoder

Based off the srt encoder. The following features are unimplemented:

- fonts, colors, sizes
- alignment and positioning

The rest works well. For example, use ffmpeg to convert subtitles into the .vtt format:

  ffmpeg -i input.srt output.vtt

Signed-off-by: Aman Gupta <ffmpeg@tmm1.net>
Signed-off-by: Clément Bœsch <u@pkh.me>
avcodec/aac: fix () in IS_CODEBOOK_UNSIGNED macro

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/golomb-test: fix () in EXTEND() macro

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/h264: fix () in macros

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/h264_slice: fix macro ()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/hevc: fix () in macros

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/ivi_dsp: add some missing () to macros

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>