scaled-scene-buffer: reduce unnecessary renderings

Prior to this commit, a backing buffer with scale 1 was always created for
a scaled_scene_buffer before showing it, and backing buffers for specific
scales were created on output_enter events.

This commit removes this redundant re-renderings by calling
wlr_scene_buffer_set_dest_size() upon scaled_scene_buffer creation just to
receive output_enter events and delaying the first rendering to the first
output_enter event.

I needed to add font_get_buffer_size() to obtain the size of a font buffer
without actually creating it.

include/common/font.h

@@ -36,6 +36,12 @@ int font_height(struct font *font);
  */
 int font_width(struct font *font, const char *string);
 
+/**
+ * font_get_buffer_size - dry-run font_buffer_create() to get buffer size
+ */
+void font_get_buffer_size(int max_width, const char *text, struct font *font,
+	int *width, int *height);
+
 /**
  * font_buffer_create - Create ARGB8888 lab_data_buffer using pango
  * @buffer: buffer pointer

include/common/scaled-scene-buffer.h

@@ -96,8 +96,17 @@ struct scaled_scene_buffer *scaled_scene_buffer_create(
 	const struct scaled_scene_buffer_impl *implementation,
 	struct wl_list *cached_buffers, bool drop_buffer);
 
-/* Clear the cache of existing buffers, useful in case the content changes */
-void scaled_scene_buffer_invalidate_cache(struct scaled_scene_buffer *self);
+/**
+ * scaled_scene_buffer_request_update - mark the buffer that needs to be
+ * updated
+ * @width: the width of the buffer to be rendered, in scene coordinates
+ * @height: the height of the buffer to be rendered, in scene coordinates
+ *
+ * This function should be called when the states bound to the buffer are
+ * updated and ready for rendering.
+ */
+void scaled_scene_buffer_request_update(struct scaled_scene_buffer *self,
+	int width, int height);
 
 /* Private */
 struct scaled_scene_buffer_cache_entry {