cairo: replace cairo with pixman in decos, particles and modules

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.

decoration.h

@@ -1,10 +1,10 @@
 #pragma once
 
-#include <cairo.h>
+#include <pixman.h>
 
 struct deco {
     void *private;
-    void (*expose)(const struct deco *deco, cairo_t *cr,
+    void (*expose)(const struct deco *deco, pixman_image_t *pix,
                    int x, int y, int width, int height);
     void (*destroy)(struct deco *deco);
 };