buffer: reduce unnecessary painting to new cairo surfaces

Add buffer_adopt_cairo_surface(), which allows wrapping an existing
cairo image surface in a struct lab_data_buffer. This is useful when
loading PNGs since most will be loaded as ARGB32 already.

Fix a memory leak in the non-ARGB32 PNG case, where we do still need to
paint to a new image surface -- we were leaking the original surface.

Eliminate an unnecessary temporary image surface in SVG loading and just
render the SVG to the image surface held by the lab_data_buffer.

I also cleaned up and clarified the buffer API a bit:

- Add a pointer to the held cairo_surface_t (so we can still access it
  if there is no cairo_t).
- Remove the free_on_destroy bool (it was always true).
- Rename unscaled_width/height to logical_width/height and add an
  explanatory comment. It was unclear what "unscaled" meant.
- Rename buffer_create_wrap() to buffer_create_from_data().

This is laying groundwork for some more icon fixes I am working on
(making sure icons are loaded and rendered at the correct scale).

src/img/img-xpm.c

@@ -387,7 +387,7 @@ pixbuf_create_from_xpm(struct file_handle *handle)
 	free(colors);
 	free(name_buf);
 
-	return buffer_create_wrap(data, w, h, 4 * w, true);
+	return buffer_create_from_data(data, w, h, 4 * w);
 
 out:
 	g_hash_table_destroy(color_hash);