gstreamer-vaapi: drop use of GSlice allocator
Some code incorrectly relied on GstVideoDecoder/Encoder base class implementation details, so need to make sure it uses same allocators.
Follow-up to !3695 (merged).
Fixes #1742 (closed).
Some code incorrectly relied on GstVideoDecoder/Encoder base class implementation details, so need to make sure it uses same allocators.
Follow-up to !3695 (merged).
Fixes #1742 (closed).