vaapivideomemory: use an image pool to cache objects.

vaapivideomemory: use an image pool to cache objects.

Use an image pool to hold VA images to be used for downloads/uploads
of contents for the associated surface.

This is an optmization for size. So, instead of creating as many VA
images as there are buffers (then VA surfaces) allocated, we only
maintain a minimal set of live VA images, thus preserving memory
resources.