Rework the way buffers are used and rendered

The current way buffers are handled is sub-optimal. They are hidden
inside QtWayland::Surface and the actual renderer, be it QtQuick or
anything else, cannot get a direct hold of them, nor it can directly
control when the underlying textures are created or deleted.

The main additions in this commit are the splitting of the QtQuick
code path and the new QWaylandBufferRef and QWaylandBufferAttacher
classes.
QWaylandBufferRef allows a renderer to retain a reference
to a wl_buffer even after the underlying Surface discarded it.
That allows the renderer to directly decide when to destroy the texture
of the buffer.
QWaylandBufferAttacher is a pure virtual class which must be implemented
by the renderer. Instances of it will be assigned to the QWaylandSurfaces,
created. Its attach() virtual method will then be called when a new buffer
is committed to the surface. The renderer can then choose to immediately
create a texture or wait for some later time. It is its responsibility to
create and destroy the GL texture, it will not happen automatically.
This functionality is implemented for QtQuick in the new QWaylandQuickCompositor
and QWaylandQuickSurface classes.

Change-Id: I6fc1dd4638dd101f6c8e22cd645653bc28830c2a