particle/empty: placeholder particle, renders nothing

But may have margins.

particles/empty.c

@@ -0,0 +1,45 @@
+#include "empty.h"
+
+#include <stdlib.h>
+
+static void
+exposable_destroy(struct exposable *exposable)
+{
+    free(exposable);
+}
+
+static int
+begin_expose(struct exposable *exposable, cairo_t *cr)
+{
+    exposable->width = exposable->particle->left_margin +
+        exposable->particle->right_margin;
+    return exposable->width;
+}
+
+static void
+expose(const struct exposable *exposable, cairo_t *cr, int x, int y, int height)
+{
+    const struct deco *deco = exposable->particle->deco;
+    if (deco != NULL)
+        deco->expose(deco, cr, x, y, exposable->width, height);
+}
+
+static struct exposable *
+instantiate(const struct particle *particle, const struct tag_set *tags)
+{
+    struct exposable *exposable = malloc(sizeof(*exposable));
+    exposable->particle = particle;
+    exposable->destroy = &exposable_destroy;
+    exposable->begin_expose = &begin_expose;
+    exposable->expose = &expose;
+    return exposable;
+}
+
+struct particle *
+particle_empty_new(int left_margin, int right_margin)
+{
+    struct particle *particle = particle_common_new(left_margin, right_margin);
+    particle->destroy = &particle_default_destroy;
+    particle->instantiate = &instantiate;
+    return particle;
+}