API: add support for buffer exports.

API: add support for buffer exports.

Add interfaces for low-level buffer exports to suport interop with
external APIs like EGL or OpenCL (OCL). Theory of operations:

- vaAcquireBufferHandle(): locks buffer for external API usage. This
  is a synchronization point. Upon successful return, useful buffer
  details are returned in a somewhat implementation-specific way to
  support interop with external APIs.

- vaReleaseBufferHandle(): unlocks buffer after usage from external
  API, and deallocates any resources that were needed.

Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>
(cherry picked from commit 544d31d2fc096a838c21f4684cec05746cfdd7b6)

Conflicts:
	va/va.h