Split BufferInterface

Currently, the BufferInterface encapsulates all the kinds of client
buffers. This has become a somewhat annoying issue as we want to
reference the shm pool if a shm buffer is destroyed, or have custom
buffer readiness logic for linux dma-buf client buffers.

Implementing all of that with the current abstractions will be
challenging as there's no good separation between different client
buffer types.

This change splits the BufferInterface class in three sub-classes -
DrmClientBuffer, LinuxDmaBufV1ClientBuffer, and ShmClientBuffer.

In addition to that, this change fixes the broken buffer ref'ing api.