Add VDPAU support

[nvllsvm]

Would help some jellyfin users.

https://github.com/jellyfin/jellyfin/issues/839

[OdinVex]

Same, some of us use AMD hardware.

[jrottenberg]

I've looked into it but it looks like a lot of effort

Version: 0.53.2
Source dir: /tmp/workdir
Build dir: /tmp/workdir/build
Build type: native build
Project name: libvdpau
Project version: undefined
C compiler for the host machine: cc (gcc 9.4.0 "cc (Ubuntu 9.4.0-1ubuntu1~20.04.1) 9.4.0")
C linker for the host machine: cc ld.bfd 2.34
C++ compiler for the host machine: c++ (gcc 9.4.0 "c++ (Ubuntu 9.4.0-1ubuntu1~20.04.1) 9.4.0")
C++ linker for the host machine: c++ ld.bfd 2.34
Host machine cpu family: x86_64
Host machine cpu: x86_64
Checking for function "secure_getenv" : YES 
Found pkg-config: /usr/bin/pkg-config (0.29.1)
Found CMake: /usr/bin/cmake (3.16.3)
Run-time dependency dri2proto found: NO (tried pkgconfig and cmake)
Run-time dependency xext found: NO (tried pkgconfig and cmake)
Configuring config.h using configuration
Program doxygen found: NO
Program dot found: NO
Run-time dependency x11 found: NO (tried pkgconfig and cmake)

src/meson.build:1:0: ERROR: Dependency "x11" not found, tried pkgconfig and cmake

so it requires :

• https://github.com/freedesktop/dri2proto/
• https://gitlab.freedesktop.org/xorg/lib/libxext

It looks like libvdpau is not required standalone and can be leverage with AV1 - ttps://www.phoronix.com/news/NVIDIA-AV1-VDPAU-FFmpeg

I've bumped libaom version to 3.4.0 see https://github.com/jrottenberg/ffmpeg/pull/356