compositor: add wlr_surface_set_preferred_buffer_scale()

References: https://gitlab.freedesktop.org/wayland/wayland/-/merge_requests/220

include/wlr/types/wlr_compositor.h

@@ -163,6 +163,8 @@ struct wlr_surface {
 	} previous;
 
 	bool opaque;
+
+	int32_t preferred_buffer_scale;
 };
 
 struct wlr_renderer;
@@ -319,6 +321,15 @@ uint32_t wlr_surface_lock_pending(struct wlr_surface *surface);
  */
 void wlr_surface_unlock_cached(struct wlr_surface *surface, uint32_t seq);
 
+/**
+ * Set the preferred buffer scale for the surface.
+ *
+ * This sends an event to the client indicating the preferred scale to use for
+ * buffers attached to this surface.
+ */
+void wlr_surface_set_preferred_buffer_scale(struct wlr_surface *surface,
+	int32_t scale);
+
 /**
  * Create the wl_compositor global, which can be used by clients to create
  * surfaces and regions.

types/wlr_compositor.c

@@ -15,7 +15,7 @@
 #include "types/wlr_subcompositor.h"
 #include "util/time.h"
 
-#define COMPOSITOR_VERSION 5
+#define COMPOSITOR_VERSION 6
 #define CALLBACK_VERSION 1
 
 static int min(int fst, int snd) {
@@ -1089,6 +1089,23 @@ void wlr_surface_get_buffer_source_box(struct wlr_surface *surface,
 	}
 }
 
+void wlr_surface_set_preferred_buffer_scale(struct wlr_surface *surface,
+		int32_t scale) {
+	assert(scale > 0);
+
+	if (wl_resource_get_version(surface->resource) <
+			WL_SURFACE_PREFERRED_BUFFER_SCALE_SINCE_VERSION) {
+		return;
+	}
+
+	if (surface->preferred_buffer_scale == scale) {
+		return;
+	}
+
+	wl_surface_send_preferred_buffer_scale(surface->resource, scale);
+	surface->preferred_buffer_scale = scale;
+}
+
 static const struct wl_compositor_interface compositor_impl;
 
 static struct wlr_compositor *compositor_from_resource(