Add a create_image_from_ptr vfunc to struct pixman_renderer_interface,
which wraps weston_renderbuffer creation for the pixman renderer via
pixman_image_create_bits(), as well as a renderbuffer_destroy vfunc
to dispose of the pixman image renderbuffer.
Also add create_image_no_clear using pixman_image_create_bits_no_clear()
instead.
Make the backends create and destroy their pixman image renderbuffers
through this interface.
Signed-off-by: Philipp Zabel <philipp.zabel@gmail.com>