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>

.forgejo/CODEOWNERS

@@ -100,6 +100,7 @@ libavfilter/.*f_ebur128.* @haasn
 libavfilter/vf_blackdetect.* @haasn
 libavfilter/vf_colordetect.* @haasn
 libavfilter/vf_colorspace.* @rbultje
+libavfilter/.*drawvg.* @ayosec
 libavfilter/vf_icc.* @haasn
 libavfilter/vf_libplacebo.* @haasn
 libavfilter/vf_premultiply.* @haasn
@@ -215,4 +216,5 @@ doc/.* @GyanD
 # tests
 # =====
 tests/checkasm/riscv/.* @Courmisch
+tests/ref/.*drawvg.* @ayosec
 tests/ref/fate/sub-mcc.* @programmerjake