swapchain: Introduce new helper for damage rings

This is advantageous over rolling your own logic because this will choose
the ideal buffer (the youngest buffer - the one with the least damage)

include/wlr/render/swapchain.h

@@ -1,12 +1,15 @@
 #ifndef WLR_RENDER_SWAPCHAIN_H
 #define WLR_RENDER_SWAPCHAIN_H
 
+#include <pixman.h>
 #include <stdbool.h>
 #include <wayland-server-core.h>
 #include <wlr/render/drm_format_set.h>
 
 #define WLR_SWAPCHAIN_CAP 4
 
+struct wlr_damage_ring;
+
 struct wlr_swapchain_slot {
 	struct wlr_buffer *buffer;
 	bool acquired; // waiting for release
@@ -29,13 +32,31 @@ struct wlr_swapchain *wlr_swapchain_create(
 	struct wlr_allocator *alloc, int width, int height,
 	const struct wlr_drm_format *format);
 void wlr_swapchain_destroy(struct wlr_swapchain *swapchain);
+
 /**
  * Acquire a buffer from the swap chain.
  *
+ * Can return NULL if the swapchain failed to acquire a buffer either because
+ * the swapchain is full, or the buffer couldn't be allocated.
+ *
  * The returned buffer is locked. When the caller is done with it, they must
  * unlock it by calling wlr_buffer_unlock.
  */
 struct wlr_buffer *wlr_swapchain_acquire(struct wlr_swapchain *swapchain);
+
+/**
+ * Acquire a buffer from the swap chain that is optimal for the damage ring.
+ * Will automatically rotate the swapchain.
+ *
+ * Can return NULL if the swapchain failed to acquire a buffer either because
+ * the swapchain is full, or the buffer couldn't be allocated.
+ *
+ * The returned buffer is locked. When the caller is done with it, they must
+ * unlock it by calling wlr_buffer_unlock.
+ */
+struct wlr_buffer *wlr_swapchain_acquire_from_damage_ring(struct wlr_swapchain *swapchain,
+	struct wlr_damage_ring *ring, pixman_region32_t *damage);
+
 /**
  * Returns true if this buffer has been created by this swapchain, and false
  * otherwise.

render/swapchain.c

@@ -3,6 +3,7 @@
 #include <wlr/util/log.h>
 #include <wlr/render/swapchain.h>
 #include <wlr/types/wlr_buffer.h>
+#include <wlr/types/wlr_damage_ring.h>
 #include "render/allocator/allocator.h"
 #include "render/drm_format_set.h"
 
@@ -108,6 +109,40 @@ struct wlr_buffer *wlr_swapchain_acquire(struct wlr_swapchain *swapchain) {
 	return slot_acquire(swapchain, free_slot);
 }
 
+struct wlr_buffer *wlr_swapchain_acquire_from_damage_ring(struct wlr_swapchain *swapchain,
+		struct wlr_damage_ring *ring, pixman_region32_t *damage) {
+	struct wlr_buffer *buffer = NULL;
+
+	struct wlr_damage_ring_buffer *entry;
+	wl_list_for_each(entry, &ring->buffers, link) {
+		for (size_t i = 0; i < WLR_SWAPCHAIN_CAP; i++) {
+			struct wlr_swapchain_slot *slot = &swapchain->slots[i];
+			if (slot->acquired) {
+				continue;
+			}
+
+			if (slot->buffer == entry->buffer) {
+				buffer = slot_acquire(swapchain, slot);
+				break;
+			}
+		}
+
+		if (buffer) {
+			break;
+		}
+	}
+
+	if (!buffer) {
+		buffer = wlr_swapchain_acquire(swapchain);
+		if (!buffer) {
+			return NULL;
+		}
+	}
+
+	wlr_damage_ring_rotate_buffer(ring, buffer, damage);
+	return buffer;
+}
+
 bool wlr_swapchain_has_buffer(struct wlr_swapchain *swapchain,
 		struct wlr_buffer *buffer) {
 	for (size_t i = 0; i < WLR_SWAPCHAIN_CAP; i++) {