The previous commit reinstantiated `girs_dir`, and this commit applies
the rename to `girs_directories` and adds the new/missing
`gst-gir-files` directory where GStreamer-specific `.gir` files live.
This reverts commit 7f9fcb09e2.
`generator.py` is in the process of being moved to a generic place in
the `gir` repository for reuse across crates. This means `-d` cannot be
passed for our GStreamer-specific `gst-gir-files` directory anymore, and
should be configured from `Gir.toml` instead.
We were already using `gir -d` and especially now that our files are
separated across two directories that are relative to the directory
containing Gir.toml this only becomes cumbersome. Besides `gir` lacks
functionality to normalize the path, leading to ie.
gstreamer-gl/egl/sys/../../../gir-files in the version comment as a
result.
{Audio,Video}Endianness is an enumeration to represent the two possible
values in Rust but this enumeration does not exist as such in C, where
it is merely an integer with some constants defining possible values.
These functions have been checked to never return NULL. The overrides
can hopefully be gone again when C contains better annotations and
trust_return_value_nullability is enabled.
