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.
The main reason why we have factory methods is that up to some point,
kwayland had its own signal to indicate when globals have to be removed.
Now that all globals add destroy listeners for the wl_display object,
we don't have that signal. Most factory methods are equivalent to doing
new T(display).
Besides adding unnecessary boilerplate code, another reason to get rid
of the factory methods is to reduce the amount of merge conflicts. If
several persons work on implementing wayland protocols at the same time,
sooner or later someone will have to resolve merge conflicts in Display.
