Add support to wl_shm for RGB565 buffers

[Tomeu Vizoso]

Hi,

this patch series allows clients to request the creation of 16bit buffers,
useful for conserving resources in devices with constrained resources when
using big surfaces such as desktop background images.

I'm asking for ideas about what to do with how renderers are now
registering wl_shm, needed because they are the ones that know what
formats are supported.

And of course, any other comments will be most welcome.

http://cgit.collabora.com/git/user/tomeu/wayland.git/log/?h=tomeu/16bpp
http://cgit.collabora.com/git/user/tomeu/weston.git/log/?h=tomeu/16bpp

Thanks,

Tomeu