scaled-scene-buffer: restructure source files

- Rename `scaled_scene_buffer` to `scaled_buffer`. This makes it clear
  that `scaled_{font,img,icon}_buffers` are implementations of it.
- Move the files from `src/common` to `src/scaled-buffer` as
  `scaled_icon_buffer` heavily depends on `server` and `view` etc.

src/scaled-buffer/scaled-img-buffer.c

@@ -0,0 +1,79 @@
+// SPDX-License-Identifier: GPL-2.0-only
+#define _POSIX_C_SOURCE 200809L
+#include "scaled-buffer/scaled-img-buffer.h"
+#include <assert.h>
+#include <wayland-server-core.h>
+#include <wlr/types/wlr_scene.h>
+#include "buffer.h"
+#include "common/mem.h"
+#include "img/img.h"
+#include "node.h"
+#include "scaled-buffer/scaled-buffer.h"
+
+static struct lab_data_buffer *
+_create_buffer(struct scaled_buffer *scaled_buffer, double scale)
+{
+	struct scaled_img_buffer *self = scaled_buffer->data;
+	struct lab_data_buffer *buffer = lab_img_render(self->img,
+		self->width, self->height, scale);
+	return buffer;
+}
+
+static void
+_destroy(struct scaled_buffer *scaled_buffer)
+{
+	struct scaled_img_buffer *self = scaled_buffer->data;
+	lab_img_destroy(self->img);
+	free(self);
+}
+
+static bool
+_equal(struct scaled_buffer *scaled_buffer_a,
+	struct scaled_buffer *scaled_buffer_b)
+{
+	struct scaled_img_buffer *a = scaled_buffer_a->data;
+	struct scaled_img_buffer *b = scaled_buffer_b->data;
+
+	return lab_img_equal(a->img, b->img)
+		&& a->width == b->width
+		&& a->height == b->height;
+}
+
+static struct scaled_buffer_impl impl = {
+	.create_buffer = _create_buffer,
+	.destroy = _destroy,
+	.equal = _equal,
+};
+
+struct scaled_img_buffer *
+scaled_img_buffer_create(struct wlr_scene_tree *parent, struct lab_img *img,
+	int width, int height)
+{
+	assert(parent);
+	assert(img);
+	assert(width >= 0 && height >= 0);
+
+	struct scaled_buffer *scaled_buffer = scaled_buffer_create(
+		parent, &impl, /* drop_buffer */ true);
+	struct scaled_img_buffer *self = znew(*self);
+	self->scaled_buffer = scaled_buffer;
+	self->scene_buffer = scaled_buffer->scene_buffer;
+	self->img = lab_img_copy(img);
+	self->width = width;
+	self->height = height;
+
+	scaled_buffer->data = self;
+
+	scaled_buffer_request_update(scaled_buffer, width, height);
+
+	return self;
+}
+
+struct scaled_img_buffer *
+scaled_img_buffer_from_node(struct wlr_scene_node *node)
+{
+	struct scaled_buffer *scaled_buffer =
+		node_scaled_buffer_from_node(node);
+	assert(scaled_buffer->impl == &impl);
+	return scaled_buffer->data;
+}