shm: Allocate shm buffers through new wl_shm_pool interface

There's a big cost to setting up and tearing down a mmap and faulting in
the pages to back it.  For cases where we're continuously reallocating
shm wl_buffers (resizing a surface, typically) it is a big performance
improvement to be able to reuse a mmap area.  This change makes the shm
buffer allocation a two step process: first allocate a wl_shm_pool, then
allocate a buffer from the pool.  The wl_shm_pool encapsulate the shared
memory pool, and lets clients allocate wl_buffers backed by chunks of that
memory.  Buffers are allocated at an offset into the pool, so it's possible
to create multiple buffers from one pool, for example for icons or cursor
images.