plugins: request GLTextureUpload meta on buffers in the buffer pool.

plugins: request GLTextureUpload meta on buffers in the buffer pool.

Requesting the GLTextureUpload meta on buffers in the bufferpool
prevents such metas from being de-allocated when buffers are released
in the sink.

This is particulary useful in terms of performance when using the
GLTextureUploadMeta API since the GstVaapiTexture associated with
the target texture is stored in the meta.

https://bugzilla.gnome.org/show_bug.cgi?id=712558

Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>