avfilter/vf_drawtext: improve glyph shaping and positioning

- text is now shaped using libharfbuz
- glyphs position is now accurate to 1/4 pixel in both directions
- the default line height is now the one defined in the font

Adds libharfbuzz dependency.

configure

@@ -236,6 +236,7 @@ External library support:
   --enable-libfontconfig   enable libfontconfig, useful for drawtext filter [no]
   --enable-libfreetype     enable libfreetype, needed for drawtext filter [no]
   --enable-libfribidi      enable libfribidi, improves drawtext filter [no]
+  --enable-libharfbuzz     enable libharfbuzz, needed for drawtext filter [no]
   --enable-libglslang      enable GLSL->SPIRV compilation via libglslang [no]
   --enable-libgme          enable Game Music Emu via libgme [no]
   --enable-libgsm          enable GSM de/encoding via libgsm [no]
@@ -1859,6 +1860,7 @@ EXTERNAL_LIBRARY_LIST="
     libfontconfig
     libfreetype
     libfribidi
+    libharfbuzz
     libglslang
     libgme
     libgsm
@@ -3726,7 +3728,7 @@ dilation_opencl_filter_deps="opencl"
 dnn_classify_filter_select="dnn"
 dnn_detect_filter_select="dnn"
 dnn_processing_filter_select="dnn"
-drawtext_filter_deps="libfreetype"
+drawtext_filter_deps="libfreetype libharfbuzz"
 drawtext_filter_suggest="libfontconfig libfribidi"
 elbg_filter_deps="avcodec"
 eq_filter_deps="gpl"
@@ -6693,6 +6695,7 @@ enabled fontconfig        && enable libfontconfig
 enabled libfontconfig     && require_pkg_config libfontconfig fontconfig "fontconfig/fontconfig.h" FcInit
 enabled libfreetype       && require_pkg_config libfreetype freetype2 "ft2build.h FT_FREETYPE_H" FT_Init_FreeType
 enabled libfribidi        && require_pkg_config libfribidi fribidi fribidi.h fribidi_version_info
+enabled libharfbuzz       && require_pkg_config libharfbuzz harfbuzz hb.h hb_buffer_create
 enabled libglslang && { check_lib spirv_compiler glslang/Include/glslang_c_interface.h glslang_initialize_process \
                             -lglslang -lMachineIndependent -lOSDependent -lHLSL -lOGLCompiler -lGenericCodeGen \
                             -lSPVRemapper -lSPIRV -lSPIRV-Tools-opt -lSPIRV-Tools -lpthread -lstdc++ -lm ||

doc/filters.texi

@@ -12344,7 +12344,7 @@ Draw a text string or text from a specified file on top of a video, using the
 libfreetype library.
 
 To enable compilation of this filter, you need to configure FFmpeg with
-@code{--enable-libfreetype}.
+@code{--enable-libfreetype} and @code{--enable-libharfbuzz}.
 To enable default font fallback and the @var{font} option you need to
 configure FFmpeg with @code{--enable-libfontconfig}.
 To enable the @var{text_shaping} option, you need to configure FFmpeg with
@@ -12372,8 +12372,7 @@ option, check the @ref{color syntax,,"Color" section in the ffmpeg-utils manual,
 The default value of @var{boxcolor} is "white".
 
 @item line_spacing
-Set the line spacing in pixels of the border to be drawn around the box using @var{box}.
+Set the line spacing in pixels. The default value of @var{line_spacing} is 0.
-The default value of @var{line_spacing} is 0.
 
 @item borderw
 Set the width of the border to be drawn around the text using @var{bordercolor}.
@@ -12873,6 +12872,9 @@ For more information about fontconfig, check:
 For more information about libfribidi, check:
 @url{http://fribidi.org/}.
 
+For more information about libharfbuzz, check:
+@url{https://github.com/harfbuzz/harfbuzz}.
+
 @section edgedetect
 
 Detect and draw edges. The filter uses the Canny Edge Detection algorithm.