Implement pixman renderer

Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
x11 backend: add option to use pixman renderer

When --use-shm is passed to weston and x11 backend is active,
it will use SHM surfaces with pixman renderer instead of EGL

Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>