server: rename globals so that they are prefixed with wl_
config: add SHM flag

Add support for wl_shm objects
server: add wl_shm global object

object and basic code for implementation
client: add binding to wl_shm global object
server: fix typo

was calling compositor_bind instead of shm_bind.
Also declare shm_bind
test: wit-test, check shm object

If it was created on both sides