Introduce libwayland-cursor, a cursor helper library

The purpose of this library is to be the equivalent of libXcursor in
the X world. This library is compatible with X cursor themes and loads
them directly into an shm pool making it easy for the clients to get
buffer for each cursor image.

The code for handling the X cursor theme was taken from libXcursor. The
files cursor/xcursor.[ch] are a stripped down version of that library
containing only the interfaces necessary for implementing the wayland
counterpart.