vaapi: declare external dependencies

vaapi: declare external dependencies

There are two main external dependencies that define the feature set of this
plugin: a) the kernel and b) the VA driver

This patch tracks both dependencies, if any of them change, GStreamer will
re-inspect the plugin.

The kernel is tracked through the device files /dev/dri/card*

The VA driver is tracked through the files VA_DRIVERS_PATH/*_drv_video.so,
where VA_DRIVERS_PATH is the one defined in libva package configuration. Also,
the environment variables LIBVA_DRIVERS_PATH and LIBVA_DRIVER_NAME are tracked
since they modify the driver lookup.

Additionally, the environment variable GST_VAAPI_ALL_DRIVERS is tracked too.

https://bugzilla.gnome.org/show_bug.cgi?id=724352