The progress-bar took *all* particle elements into account when
calculating the (relative) mouse position on mouse events.
This is wrong, since only the progress bar (that is, the empty and
filled cells) should be used. Not the start/end elements.
We now also pass the mouse event through to the start/end elements.
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.
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.
