buffer: drop parent meta in deep copy

The purpose of a deep buffer copy is to be able to release the source
buffer and all its dependencies. Attaching the parent buffer meta to
the newly created deep copy needlessly keeps holding a reference to the
parent buffer.

Without this patch, a v4l2h264dec ! glimagesink pipeline fails to decode
h.264 streams that change format mid-stream because the glimagesink
fails to release all DMABuf buffers to the v4l2videodec when asked to,
even though it tries by making a deep copy of its last buffer and then
releasing it:

 LOG               GST_BUFFER gstbuffer.c:799:gst_buffer_new: new 0x73b19408
 LOG               GST_BUFFER gstbuffer.c:515:gst_buffer_copy_into: copy 0x73b18d28 to 0x73b19408, offset 0-114688/114688
 LOG            glbasetexture gstglmemory.c:159:_calculate_unpack_length: Found alignment of 8 based on width (with plane width:224, plane stride:896 and pixel stride:4. RU8(224*4) = 896)
 DEBUG           glbasememory gstglbasememory.c:178:gst_gl_base_memory_init: new GL buffer memory:0x7250bc40 size:114688
 DEBUG          glbasetexture gstglmemory.c:340:gst_gl_memory_init: new GL texture context:<glcontextegl0> memory:0x7250bc40 target:2D format:6408 dimensions:224x128 stride:896 size:114688
 LOG             glbasememory gstglbasememory.c:259:_map_data_gl: mapping mem 0x7250bc40 flags 20002
 LOG             glbasememory gstglbasememory.c:259:_map_data_gl: mapping mem 0x728e3e78 flags 20001
 LOG            glbasetexture gstglmemory.c:676:gst_gl_memory_copy_teximage: copying memory 0x728e3e78, tex 2 into texture 146
 LOG             glbasememory gstglbasememory.c:338:_unmap_data_gl: unmapping mem 0x728e3e78 flags 20001
 LOG             glbasememory gstglbasememory.c:338:_unmap_data_gl: unmapping mem 0x7250bc40 flags 20002
 LOG               GST_BUFFER gstbuffer.c:414:_memory_add: buffer 0x73b19408, idx -1, mem 0x7250bc40
 DEBUG             GST_BUFFER gstbuffer.c:2201:gst_buffer_add_meta: alloc metadata 0x73b1747c (GstParentBufferMeta) of size 12
 DEBUG             GST_BUFFER gstbuffer.c:2201:gst_buffer_add_meta: alloc metadata 0x73b1b044 (GstGLSyncMeta) of size 52
 LOG               glsyncmeta gstglsyncmeta.c:300:_gst_gl_sync_meta_transform: copying sync meta 0x72506484 into 0x73b1b044
 LOG               glsyncmeta gstglsyncmeta.c:108:_default_copy: copy sync object (nil) from meta 0x72506484 to 0x73b1b044
 LOG               glsyncmeta gstglsyncmeta.c:188:_set_sync_point: setting sync point 0x72506484
 DEBUG             GST_BUFFER gstbuffer.c:2201:gst_buffer_add_meta: alloc metadata 0x73b1b084 (GstGLSyncMeta) of size 52
 LOG               glsyncmeta gstglsyncmeta.c:300:_gst_gl_sync_meta_transform: copying sync meta 0x73b16d4c into 0x73b1b084
 LOG               glsyncmeta gstglsyncmeta.c:108:_default_copy: copy sync object (nil) from meta 0x73b16d4c to 0x73b1b084
 LOG               glsyncmeta gstglsyncmeta.c:188:_set_sync_point: setting sync point 0x73b16d4c
 DEBUG             GST_BUFFER gstbuffer.c:2201:gst_buffer_add_meta: alloc metadata 0x73b0190c (GstVideoMeta) of size 76
 LOG               GST_BUFFER gstbuffer.c:710:_gst_buffer_dispose: release 0x73b18d28 to pool 0x73b230e0
 DEBUG             GST_BUFFER gstbuffer.c:2383:gst_buffer_foreach_meta: remove metadata 0x73206484 (GstParentBufferMeta)
 DEBUG             GST_BUFFER gstbuffer.c:2383:gst_buffer_foreach_meta: remove metadata 0x72506484 (GstGLSyncMeta)
 LOG               glsyncmeta gstglsyncmeta.c:316:_free_gl_sync_meta: free sync meta 0x72506484
 LOG               bufferpool gstbufferpool.c:1284:default_release_buffer:<glbufferpool0> released buffer 0x73b18d28 0

Note that the GL buffer is released, but not the parent DMABuf, which
is added to the new copy via GstParentBufferMeta. It is outstanding from
the v4l2videodec src bufferpool and keeps it from stopping properly:

 DEBUG             GST_BUFFER gstbuffer.c:1375:gst_buffer_is_memory_range_writable: idx 0, length -1
 ERROR           v4l2videodec gstv4l2videodec.c:271:gst_v4l2_video_dec_set_format:<v4l2h264dec0> Stop capture

 DEBUG                   v4l2 gstv4l2object.c:4326:gst_v4l2_object_stop:<v4l2h264dec0:src> stopping
 DEBUG                   v4l2 gstv4l2object.c:4334:gst_v4l2_object_stop:<v4l2h264dec0:src> deactivating pool
 LOG               bufferpool gstbufferpool.c:506:gst_buffer_pool_set_active:<v4l2h264dec0:pool:src> active 0
 LOG               bufferpool gstbufferpool.c:538:gst_buffer_pool_set_active:<v4l2h264dec0:pool:src> outstanding buffers 1

After this, the format change and stream restart fails in the
v4l2videodec.
8 jobs for no-copy-meta in 55 minutes and 11 seconds (queued for 41 minutes and 19 seconds)
latest
Status Job ID Name Coverage
  Preparation
passed #154768
gst indent

00:00:58

passed #154767
manifest

00:01:00

 
  Build
passed #154769
build fedora x86_64

00:06:23

 
  Test
passed #154770
check fedora

00:04:32

 
  Full Builds
passed #154771
build android arm64 api28

00:05:02

passed #154773
build cerbero cross win32

00:14:54

passed #154774
build cerbero cross win64

00:09:44

passed #154772
build cerbero fedora x86_64

00:12:33