Fix deleting GraphicsBuffer twice

When GraphicsBuffer::dropped() is emitted, the buffer can be
unreferenced. If that's the case, the GraphicsBuffer will be deleted
twice: first in GraphicsBuffer::unref(), the second time in drop().

In order to address the issue, this change gets rid of the
GraphicsBuffer::dropped() signal, so it's always guaranteed that the
buffer stays alive until the reference count is checked.

The GraphicsBuffer::dropped() signal is used to remove the mapping
between wl_resource and ShmClientBuffer when the corresponding
wl_shm_buffer object is destroyed. On the other hand, we could perform
such cleanup when calling drop() too. This code can be further improved
by reimplementing wl-shm, which we need to do at some point in the
future.