Prior to this commit, MetaWaylandSurface held a reference to
MetaWaylandBuffer, who owned the texture drawn by the surface. When
switching buffer, the texture change with it.

This is problematic when dealing with SHM buffer damage management, as
when having one texture per buffer, damaged regions uploaded to one,
will not follow along to the next one attached. It also wasted GPU
memory as there would be one texture per buffer, instead of one one
texture per surface.

Instead, move the texture ownership to MetaWaylandSurface, and have the
SHM buffer damage management update the surface texture. This ensures
damage is processed properly, and that we won't end up with stale
texture content when doing partial texture uploads. If the same SHM
buffer is attached to multiple surfaces, each surface will get their own
copy, and damage is tracked and uploaded separately.

Non-SHM types of buffers still has their own texture reference, as the
texture is just a representation of the GPU memory associated with the
buffer. When such a buffer is attached to a surface, instead the surface
just gets a reference to that texture, instead of a separately allocated
one.

Fixes: #199