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.
Since this struct only contained function pointers, make all particles
export those functions directly.
The plugin manager now defines a particle interface struct, and fills
it it by dlsym:ing the functions that used to be in particle_info.
This is intended to be a format-like string, with the possibility to
use tag formatters.
The expanded string will later be passed to the system() call.
