Extensions and layers can be enabled before calling
gst_vulkan_instance_open() but after calling
gst_vulkan_instance_fill_info().
Use the list of available extensions to better choose a default display
implementation to use based on the available Vulkan extensions for surface
output.
Defaults are still the same.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/1341>
By passing NULL to `g_signal_new` instead of a marshaller, GLib will
actually internally optimize the signal (if the marshaller is available
in GLib itself) by also setting the valist marshaller. This makes the
signal emission a bit more performant than the regular marshalling,
which still needs to box into `GValue` and call libffi in case of a
generic marshaller.
Note that for custom marshallers, one would use
`g_signal_set_va_marshaller()` with the valist marshaller instead.
