v4l2videoenc: always allocate CAPTURE buffer from our pool

The videoencoder base class always uses the negotiated allocator for allocating coded buffers and ignores the negotiated buffer pool. Therefore, the v4l2videoenc always has to copy buffers from the pool into the allocated output buffers.

This breaks downstream elements that want to import the CAPTURE buffers of the v4l2videoenc, since the v4l2videoenc copies the exported the CAPTURE buffers and sends the copies downstream.

Always use the CAPTURE buffer pool for acquiring CAPTURE buffers instead of allocating the buffers in the base class.

Fixes: #2387 (closed)