Refactor the GstVaapiCodedBuffer APIs so that to more clearly separate
public and private interfaces. Besides, the map/unmap APIs should not
be exposed as is but appropriate accessors should be provided instead.

* GstVaapiCodedBuffer: VA coded buffer abstraction
- gst_vaapi_coded_buffer_get_size(): get coded buffer size.
- gst_vaapi_coded_buffer_copy_into(): copy coded buffer into GstBuffer

* GstVaapiCodedBufferPool: pool of VA coded buffer objects
- gst_vaapi_coded_buffer_pool_new(): create a pool of coded buffers of
  the specified max size, and bound to the supplied encoder

* GstVaapiCodedBufferProxy: pool-allocated VA coded buffer object proxy
- gst_vaapi_coded_buffer_proxy_new_from_pool(): create coded buf from pool
- gst_vaapi_coded_buffer_proxy_get_buffer(): get underlying coded buffer
- gst_vaapi_coded_buffer_proxy_get_buffer_size(): get coded buffer size

Rationale: more optimized transfer functions might be provided in the
future, thus rendering the map/unmap mechanism obsolete or sub-optimal.

https://bugzilla.gnome.org/show_bug.cgi?id=719775