Using a lookup table, try to map the user-provided xcursor string to a
cursor-shape-v1 known shape.
If we succeed, set the user’s custom cursor using server side
cursors (i.e. using cursor-shape-v1).
If not, fallback to trying to load the image ourselves (using
wl_cursor_theme_get_cursor()), and set it using the legacy
wl_pointer_set_cursor().
This implements support for the new cursor-shape-v1 protocol. When
available, we use it, instead of client-side cursor surfaces, to
select the xcursor shape.
Note that we still need to keep client side pointers, for:
* backward compatibility
* to be able to "hide" the cursor
Closes#1379
