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.
The visual interface was meant to be a generic mechanism for
specifying the content of a buffer. It goes back to before we had the
buffer factory interfaces (like wl_drm and wl_shm) and we wanted to
keep it open-ended enough that yuv, png or even svg buffer or so would
be possible.
Now that we have the buffer abstraction, we can add different buffer
types by introducing new interfaces that create buffers. It only
makes sense to leave it to those interfaces to specify the contents of
the buffers.
For wl_shm, this means that we now just specify the pixel format using
an enum. For EGL buffers, the exact pixel formats are controlled by
the implementation (part of wl_drm and similar), and from the client
point of view, everything is controlled using EGLConfigs.
