shm: auto-purge when we have multiple buffers eligible for re-use

It may happen that we end up with multiple non-busy, same-sized
buffers for the same cookie (context), and thus eligible for re-use.

Before this patch, we would keep all those buffers around. This is
completely unnecessary. Under normal circumstances, we’ll either be
re-using a single buffer, or swap between two. In the second case, the
“other” buffer is always busy, and thus not eligible for re-use.

So, if we _do_ detect multiple, re-usable buffers, pick the one with
the lowest “age” (increasing the chance of applying damage tracking,
instead of re-drawing everything), and mark the other one for purging.

shm.c

@@ -494,6 +494,15 @@ shm_get_buffer(struct wl_shm *shm, int width, int height, unsigned long cookie,
                     it->item.scroll_damage = NULL;
                     xassert(it->item.pix_instances == pix_instances);
                     cached = &it->item;
+                } else {
+                    /* We have multiple buffers eligable for
+                     * re-use. Pick the “youngest” one, and mark the
+                     * other one for purging */
+                    if (it->item.age < cached->age) {
+                        cached->purge = true;
+                        cached = &it->item;
+                    } else
+                        it->item.purge = true;
                 }
             }
     }