ffmpeg: add support for libass

Requested in https://github.com/termux/termux-packages/issues/2399.

packages/ffmpeg/build.sh

@@ -3,13 +3,10 @@ TERMUX_PKG_DESCRIPTION="Tools and libraries to manipulate a wide range of multim
 TERMUX_PKG_LICENSE="GPL-3.0"
 # NOTE: mpv has to be rebuilt and version bumped after updating ffmpeg.
 TERMUX_PKG_VERSION=4.1.3
-TERMUX_PKG_REVISION=3
-TERMUX_PKG_SHA256=0c3020452880581a8face91595b239198078645e7d7184273b8bcc7758beb63d
+TERMUX_PKG_REVISION=4
 TERMUX_PKG_SRCURL=https://www.ffmpeg.org/releases/ffmpeg-${TERMUX_PKG_VERSION}.tar.xz
-# libbz2 is used by matroska decoder:
-# libvpx is the VP8 & VP9 video encoder for WebM, see
-# https://trac.ffmpeg.org/wiki/Encode/VP8 and https://trac.ffmpeg.org/wiki/Encode/VP9
-TERMUX_PKG_DEPENDS="libbz2, libiconv, libsoxr, libx264, libx265, xvidcore, libvorbis, libmp3lame, libopus, libvpx, libgnutls, libandroid-glob, freetype, zlib"
+TERMUX_PKG_SHA256=0c3020452880581a8face91595b239198078645e7d7184273b8bcc7758beb63d
+TERMUX_PKG_DEPENDS="libass, libbz2, libiconv, libsoxr, libx264, libx265, xvidcore, libvorbis, libmp3lame, libopus, libvpx, libgnutls, libandroid-glob, freetype, zlib"
 TERMUX_PKG_INCLUDE_IN_DEVPACKAGE="share/ffmpeg/examples"
 TERMUX_PKG_CONFLICTS="libav"
 
@@ -54,6 +51,7 @@ termux_step_configure() {
 		--enable-cross-compile \
 		--enable-gnutls \
 		--enable-gpl \
+		--enable-libass \
 		--enable-libmp3lame \
 		--enable-libfreetype \
 		--enable-libvorbis \