When importing the prelude of a crate like `gst` the `glib` prelude is
provided too. Shedding these imports saves quite a few lines and
adheres to keeping it simple; we're not reexporting base/parent preludes
for no reason :)
GLMemory input/output objects cannot be passed as mutable:
to_glib_none_mut checks if the miniobject is writable, even though the
underlying implementation should really only care about whether the
_data_ owned by this miniobject is.
(This temporary GLMemory object references its parent buffer object
that is also referenced externally, resulting in two references making
the object itself immutable)
Furthermore take care of automatically calling `add_rgba_pad_templates`
in `class_init` if `ADD_RGBA_PAD_TEMPLATES` is set to `true` (the
default).
