This function behaves identically to `scaled_font_buffer_update()`
but allows setting the text as pango markup, supporting further
customization like underscores.
- 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.