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/common/font.c

@@ -109,7 +109,7 @@ font_buffer_create(struct lab_data_buffer **buffer, int max_width,
 	}
 
 	*buffer = buffer_create_cairo(text_extents.width + arrow_extents.width,
-			text_extents.height, scale, true);
+			text_extents.height, scale);
 	if (!*buffer) {
 		wlr_log(WLR_ERROR, "Failed to create font buffer");
 		return;