All decoration, particle and module interfaces now takes a
pixman_image_t parameter, and all drawing is done using pixman APIs.
The wayland/xcb backends implement a new interface functions,
get_pixman_image(), that should return a pixman image instance that is
suitable for rendering.
In the wayland backend, the image uses the same backing data as the
cairo surface.
In the XCB backend, we create a new image each time, and then blit it
to the cairo surface at commit time.
In cases where it makes sense, use calloc() instead of malloc():
* When allocating large objects with many members, many for which
NULL/0 is a good default value.
* Arrays etc where we explicitly initialize to NULL anyway.
