drawtext: Add fontconfig support

Introduce the `font` option and make it optional to pass a fontfile.

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
Merge commit 'f1b239ec8b38474d31f3bfea1f0b1693774fa432'

* commit 'f1b239ec8b38474d31f3bfea1f0b1693774fa432':
  drawtext: Add fontconfig support

Conflicts:
	configure
	doc/filters.texi
	libavfilter/vf_drawtext.c

See: 40b7a27bad8b8b35cb215bf4f4cba02a00d6f9e2
Merged-by: Michael Niedermayer <michaelni@gmx.at>
x86: move horizontal add macros to x86util

Also port relevant AVX2/XOP optimizations from x264 with permission
to relicense to LGPL from the corresponding authors

Signed-off-by: James Almer <jamrial@gmail.com>
Reviewed-by: "Ronald S. Bultje" <rsbultje@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
x86/dsputil: use HADDD where applicable

Signed-off-by: James Almer <jamrial@gmail.com>
Reviewed-by: "Ronald S. Bultje" <rsbultje@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>