Add hardware decoding support on Linux platforms (VA-API, VDPAU)

[FreezyLemon]

The libraries necessary to enable hwaccel on Linux (libva, libvdpau) are required by FFmpeg, but probably shouldn't be bundled with the game.

One good idea by @bdach was to bundle FFmpeg libraries without hardware acceleration, and to modify the library loading logic so that FFmpeg libraries installed on the system are preferred over the bundled ones.

That way, if a user installs FFmpeg with hwaccel support for their specific machine, it'll load those libraries (= hardware decoding) and falls back to the bundled libraries (= software decoding) when no installed FFmpeg can be found.