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.

include/scaled-buffer/scaled-font-buffer.h

@@ -0,0 +1,84 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef LABWC_SCALED_FONT_BUFFER_H
+#define LABWC_SCALED_FONT_BUFFER_H
+
+#include "common/font.h"
+
+struct wlr_scene_tree;
+struct wlr_scene_buffer;
+struct scaled_buffer;
+
+struct scaled_font_buffer {
+	struct wlr_scene_buffer *scene_buffer;
+	int width;   /* unscaled, read only */
+	int height;  /* unscaled, read only */
+
+	/* Private */
+	char *text;
+	int max_width;
+	float color[4];
+	float bg_color[4];
+	struct font font;
+	struct scaled_buffer *scaled_buffer;
+
+	/*
+	 * The following fields are used only for the titlebar, where
+	 * the font buffer can be rendered with a pattern background to
+	 * support gradients. In this case, the font buffer is also
+	 * padded to a fixed height (with the text centered vertically)
+	 * in order to align the pattern with the rest of the titlebar.
+	 */
+	int fixed_height;
+	cairo_pattern_t *bg_pattern; /* overrides bg_color if set */
+};
+
+/**
+ * Create an auto scaling font buffer, providing a wlr_scene_buffer node for
+ * display. It gets destroyed automatically when the backing scaled_buffer
+ * is being destroyed which in turn happens automatically when the backing
+ * wlr_scene_buffer (or one of its parents) is being destroyed.
+ *
+ * To actually show some text, scaled_font_buffer_update() has to be called.
+ *
+ */
+struct scaled_font_buffer *scaled_font_buffer_create(struct wlr_scene_tree *parent);
+
+/**
+ * Create an auto scaling font buffer for titlebar text.
+ * The font buffer takes a new reference to bg_pattern.
+ *
+ * @param fixed_height Fixed height for the buffer (logical pixels)
+ * @param bg_pattern Background pattern (solid color or gradient)
+ */
+struct scaled_font_buffer *
+scaled_font_buffer_create_for_titlebar(struct wlr_scene_tree *parent,
+	int fixed_height, cairo_pattern_t *bg_pattern);
+
+/**
+ * Update an existing auto scaling font buffer.
+ *
+ * No steps are taken to detect if its actually required to render a new buffer.
+ * This should be done by the caller to prevent useless recreation of the same
+ * buffer in case nothing actually changed.
+ *
+ * Some basic checks could be something like
+ * - truncated = buffer->width == max_width
+ * - text_changed = strcmp(old_text, new_text)
+ * - font and color the same
+ *
+ * bg_color is ignored for font buffers created with
+ * scaled_font_buffer_create_for_titlebar().
+ */
+void scaled_font_buffer_update(struct scaled_font_buffer *self, const char *text,
+	int max_width, struct font *font, const float *color,
+	const float *bg_color);
+
+/**
+ * Update the max width of an existing auto scaling font buffer
+ * and force a new render.
+ *
+ * No steps are taken to detect if its actually required to render a new buffer.
+ */
+void scaled_font_buffer_set_max_width(struct scaled_font_buffer *self, int max_width);
+
+#endif /* LABWC_SCALED_FONT_BUFFER_H */