`BaseTransformImpl` leak when returning `gst_base::BASE_TRANSFORM_FLOW_DROPPED`

Describe your issue

Hi all! I try to implement a filtering element that drops buffers based on some condition. But I faced a memory leak, so I've made a minimal example to reproduce this: https://github.com/DmitrySamoylov/gst-rs-drop-buffers-example

impl BaseTransformImpl for Filter {
    const MODE: BaseTransformMode = BaseTransformMode::AlwaysInPlace;
    const PASSTHROUGH_ON_SAME_CAPS: bool = false;
    const TRANSFORM_IP_ON_PASSTHROUGH: bool = false;

    fn transform_ip(&self, _buf: &mut gst::BufferRef) -> Result<gst::FlowSuccess, gst::FlowError> {
        Ok(gst_base::BASE_TRANSFORM_FLOW_DROPPED)
    }
}

Expected Behavior

I guess the buffer must be dropped automatically by GStreamer when transform_ip() returns gst_base::BASE_TRANSFORM_FLOW_DROPPED.

Observed Behavior

The buffer is not dropped according to logs:

export GST_DEBUG="2,GST_BUFFER:7"
export GST_PLUGIN_PATH="target/debug/"
gst-launch-1.0 videotestsrc num-buffers=2 ! myfilter ! fakesink

Setting pipeline to PAUSED ...
Pipeline is PREROLLING ...
0:00:00.019201723 65825 0x5628316f5aa0 ERROR               GST_RUST base_transform.rs:684:<_ as gstreamer_base::subclass::base_transform::BaseTransformImplExt>::parent_propose_allocation::{{closure}}:<filter0> Parent function `propose_allocation` failed
0:00:00.019332462 65825 0x5628316f5aa0 LOG               GST_BUFFER gstbuffer.c:844:gst_buffer_new: new 0x5628319705a0
0:00:00.019345227 65825 0x5628316f5aa0 LOG               GST_BUFFER gstbuffer.c:450:_memory_add: buffer 0x5628319705a0, idx -1, mem 0x7fab9c00cde0
0:00:00.019354053 65825 0x5628316f5aa0 LOG               GST_BUFFER gstbuffer.c:898:gst_buffer_new_allocate: new buffer 0x5628319705a0 of size 115200 from allocator (nil)
0:00:00.019368321 65825 0x5628316f5aa0 LOG               GST_BUFFER gstbuffer.c:1800:gst_buffer_map_range: buffer 0x5628319705a0, idx 0, length -1, flags 0002
0:00:00.019379071 65825 0x5628316f5aa0 LOG               GST_BUFFER gstbuffer.c:244:_get_merged_memory: buffer 0x5628319705a0, idx 0, length 1
0:00:00.020978092 65825 0x5628316f5aa0 LOG               GST_BUFFER gstbuffer.c:844:gst_buffer_new: new 0x5628319706c0
0:00:00.020991097 65825 0x5628316f5aa0 LOG               GST_BUFFER gstbuffer.c:450:_memory_add: buffer 0x5628319706c0, idx -1, mem 0x7fab9c056d00
0:00:00.020998221 65825 0x5628316f5aa0 LOG               GST_BUFFER gstbuffer.c:898:gst_buffer_new_allocate: new buffer 0x5628319706c0 of size 115200 from allocator (nil)
0:00:00.021011526 65825 0x5628316f5aa0 LOG               GST_BUFFER gstbuffer.c:1800:gst_buffer_map_range: buffer 0x5628319706c0, idx 0, length -1, flags 0002
0:00:00.021019351 65825 0x5628316f5aa0 LOG               GST_BUFFER gstbuffer.c:244:_get_merged_memory: buffer 0x5628319706c0, idx 0, length 1
Pipeline is PREROLLED ...
Setting pipeline to PLAYING ...
New clock: GstSystemClock
Got EOS from element "pipeline0".
Execution ended after 0:00:00.000066838
Setting pipeline to NULL ...
Freeing pipeline ...

I expect to see calls to _gst_buffer_free/_gst_buffer_dispose.

Setup

• Operating System: Ubuntu 20.04
• Device: Computer
• gstreamer-rs Version: 0.20.6
• GStreamer Version: 1.16.3

Steps to reproduce the bug

git clone https://github.com/DmitrySamoylov/gst-rs-drop-buffers-example
cd gst-rs-drop-buffers-example
./runme.sh

How reproducible is the bug?

Always