Redesign client buffer management

With the current design, both kwayland-server and kwin are involved in
getting opengl textures from client buffers. This is somewhat goes
against the purpose of kwayland-server, which is to simplify writing
wayland compositors. Dealing with EGLImage or linux dmabuf buffers is
not difficult, but it involves a lot of boilerplate code, which ideally
needs to be in a library.

Another confusing thing about our current buffer abstractions is that
referenced buffers can be destroyed. It makes some parts of kwin hard to
understand. The whole purpose of the ref'ing is to ensure that nothing
will be destroyed while it's being used by the compositor. Furthermore,
buffers are not automatically ref'ed/unref'ed.

In order to keep things extensible and less error prone, this change
rewrite the buffer management bits so the compositors don't have to
implement their own code path for getting OpenGL textures from client
buffers. This is also a necessary step towards asynchronous uploading
of data from shared memory buffers to textures and proper implementation
of the linux explicit synchronization protocol.

With the new design, the most important classes are ClientBufferRef and
ClientBuffer.

The ClientBuffer is an internal type, and it represents the actual
buffer. The ClientBufferRef is a reference to a ClientBuffer. As long as
there is a reference, the associated buffer cannot be modified by the
client.

ClientBufferRef acts as a "smart pointer." It will keep the reference
count up to date as you copy or move ClientBufferRefs objects. If the
refcount reaches 0, the buffer will be released automatically.

In the event of the underlying buffer resource getting destroyed, for
example if the app has crashed, the corresponding ClientBuffer will not
be destroyed. All existing references will remain valid.

Some plugins, e.g. linux-dmabuf-v1, need an EGLDisplay or a way to get
notified when a graphical reset has occurred. For this particular reason
the RendererInterface class has been introduced that acts as a bridge
between the renderer and the rest of buffer management.

Note that the wayland-eglstream plugin is licensed under GPL-2.0+
because some of its code was copied from kwin.
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