fix(linux): restore VAAPI hardware decode and AV1 software fallback in the bundled libmpv

Hardware decoding stopped working for Linux users on 2.13.0 (Fedora 44
report): every source decodes in software, and AV1 plays black video with
audio. Two defects in the pinned libmpv build.

First, mpv's meson 'drm' feature silently disabled itself because the CI
builder lacks libdisplay-info, and every VAAPI path that does not depend
on a display server is derived from it: vaapi-copy's standalone render-node
device (the path 2.12.1 worked on) and the GL dmabuf interop for direct
vaapi. With only the Wayland VA provider compiled in, a machine whose
Wayland VA display fails to initialize has no fallback, and vaapi-copy
has an empty provider list - every source lands on software decoding.
Pin -Ddrm=enabled, -Dvaapi-drm=enabled, -Degl=enabled and
-Dvaapi-wayland=enabled, and add libdisplay-info-dev to the CI package
lists, so a missing piece fails the build instead of shipping silent
software decode.

Second, the bundled static FFmpeg has no AV1 software decoder: its native
av1 codec is hardware-accelerated only, so once hwdec fails there is no AV1
path at all - every packet errors, video hits EOF, the plane goes black
while audio keeps playing. Pin dav1d 1.5.4 (both VideoLAN remotes agree on
the tag object and root commit), build it static before ffmpeg, and pass
--enable-libdav1d.

The build-plan stub test now asserts the hwdec feature flags, the dav1d
static build, and ffmpeg's libdav1d. Verified in an ubuntu:24.04 container
with the production flag sets: meson reports drm, vaapi-drm, vaapi-wayland,
egl and dmabuf-interop-gl enabled, and ffmpeg configures CONFIG_LIBDAV1D=yes
with the AV1 VAAPI hwaccel.

close #1874

.github/workflows/build.yml

@@ -36,7 +36,7 @@ env:
     libharfbuzz-dev libepoxy-dev libegl-dev libgl-dev libgnutls28-dev libpipewire-0.3-dev
     libva-dev libvdpau-dev libx11-dev libxext-dev libxrandr-dev libxcursor-dev libxi-dev
     libxss-dev libxpresent-dev libxkbcommon-dev libpulse-dev libdbus-1-dev libdrm-dev
-    libgbm-dev libwayland-dev wayland-protocols liblcms2-dev libmujs-dev liblua5.2-dev
+    libdisplay-info-dev libgbm-dev libwayland-dev wayland-protocols liblcms2-dev libmujs-dev liblua5.2-dev
     ruby ruby-dev rubygems build-essential rpm libarchive-tools imagemagick libcurl4-openssl-dev
 
 jobs:

.github/workflows/ci.yml

@@ -657,7 +657,7 @@ jobs:
             libasound2-dev libass-dev libfreetype-dev libfontconfig-dev libfribidi-dev \
             libharfbuzz-dev libegl-dev libgl-dev libgnutls28-dev libpipewire-0.3-dev \
             libva-dev libxkbcommon-dev libpulse-dev libdbus-1-dev libdrm-dev \
-            libgbm-dev libwayland-dev wayland-protocols liblcms2-dev libmujs-dev \
+            libdisplay-info-dev libgbm-dev libwayland-dev wayland-protocols liblcms2-dev libmujs-dev \
             liblua5.2-dev rpm libarchive-tools imagemagick ruby-dev build-essential
           sudo gem install fpm --version 1.17.0 --no-document