render: introduce wlr_swapchain

The swapchain maximum capacity is set to 4, so that we have enough room
for:

- A buffer currently displayed on screen
- A buffer queued for display (e.g. to KMS)
- A pending buffer that'll be queued next commit
- An additional pending buffer in case we want to invalidate the
  currently pending one

include/render/swapchain.h

@@ -0,0 +1,41 @@
+#ifndef RENDER_SWAPCHAIN_H
+#define RENDER_SWAPCHAIN_H
+
+#include <stdbool.h>
+#include <wayland-server-core.h>
+#include <wlr/render/drm_format_set.h>
+
+#define WLR_SWAPCHAIN_CAP 4
+
+struct wlr_swapchain_slot {
+	struct wlr_buffer *buffer;
+	bool acquired; // waiting for release
+
+	struct wl_listener release;
+};
+
+struct wlr_swapchain {
+	struct wlr_allocator *allocator; // NULL if destroyed
+
+	int width, height;
+	struct wlr_drm_format *format;
+
+	struct wlr_swapchain_slot slots[WLR_SWAPCHAIN_CAP];
+
+	struct wl_listener allocator_destroy;
+};
+
+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.
+ *
+ * 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);
+
+#endif