lavfi: add drawvg video filter.

The drawvg filter can draw vector graphics on top of a video, using libcairo. It
is enabled if FFmpeg is configured with `--enable-cairo`.

The language for drawvg scripts is documented in `doc/drawvg-reference.texi`.

There are two new tests:

- `fate-filter-drawvg-interpreter` launch a script with most commands, and
  verify which libcairo functions are executed.
- `fate-filter-drawvg-video` render a very simple image, just to verify that
  libcairo is working as expected.

Signed-off-by: Ayose <ayosec@gmail.com>

doc/Makefile

@@ -28,6 +28,7 @@ HTMLPAGES   = $(AVPROGS-yes:%=doc/%.html) $(AVPROGS-yes:%=doc/%-all.html) $(COMP
               doc/mailing-list-faq.html                                 \
               doc/nut.html                                              \
               doc/platform.html                                         \
+              doc/drawvg-reference.html                                 \
               $(SRC_PATH)/doc/bootstrap.min.css                         \
               $(SRC_PATH)/doc/style.min.css                             \
               $(SRC_PATH)/doc/default.css                               \

doc/filters.texi

@@ -13049,6 +13049,78 @@ For more information about libfribidi, check:
 For more information about libharfbuzz, check:
 @url{https://github.com/harfbuzz/harfbuzz}.
 
+@anchor{drawvg}
+@section drawvg
+
+Draw vector graphics on top of video frames, by executing a script written in
+a custom language called VGS (@emph{Vector Graphics Script}).
+
+The documentation for the language can be found in
+@ref{,,drawvg - Language Reference,drawvg-reference}.
+
+Graphics are rendered using the @uref{https://cairographics.org/,cario 2D
+graphics library}.
+
+To enable compilation of this filter, you need to configure FFmpeg with
+@code{--enable-cairo}.
+
+@subsection Parameters
+
+Either @code{script} or @code{file} must be set.
+
+@table @option
+
+@item s, script
+Script source to draw the graphics.
+
+@item file
+Path of the file to load the script source.
+
+@end table
+
+@subsection Pixel Formats
+
+Since Cairo only supports RGB images, if the input video is something else (like
+YUV 4:2:0), before executing the script the video is converted to a format
+compatible with Cairo. Then, you have to use use either the @ref{format} filter,
+or the @code{-pix_fmt} option, to convert it to the expected format in the
+output.
+
+@subsection Examples
+
+@itemize
+@item
+Draw the outline of an ellipse.
+
+@example
+ffmpeg -i input.webm \
+    -vf 'drawvg=ellipse (w/2) (h/2) (w/3) (h/3) stroke' \
+    -pix_fmt yuv420p \
+    output.webm
+@end example
+
+@item
+
+Draw a square rotating in the middle of the frame.
+
+The script for drawvg is in a file @code{draw.vgs}:
+
+@example
+translate (w/2) (h/2)
+rotate t
+rect -100 -100 200 200
+setcolor red@@0.5
+fill
+@end example
+
+Then:
+
+@example
+ffmpeg -i input.webm -vf 'drawvg=file=draw.vgs,format=yuv420p' output.webm
+@end example
+
+@end itemize
+
 @section edgedetect
 
 Detect and draw edges. The filter uses the Canny Edge Detection algorithm.