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-xbm.c

@@ -266,8 +266,8 @@ img_xbm_from_bitmap(const char *bitmap, struct lab_data_buffer **buffer,
 	}
 	color = argb32(rgba);
 	pixmap = parse_xbm_builtin(bitmap, 6);
-	*buffer = buffer_create_wrap(pixmap.data, pixmap.width, pixmap.height,
-		pixmap.width * 4, /* free_on_destroy */ true);
+	*buffer = buffer_create_from_data(pixmap.data, pixmap.width, pixmap.height,
+		pixmap.width * 4);
 }
 
 void
@@ -298,9 +298,9 @@ img_xbm_load(const char *filename, struct lab_data_buffer **buffer,
 		return;
 	}
 
-	/* Create buffer with free_on_destroy being true */
+	/* Create buffer */
 	if (pixmap.data) {
-		*buffer = buffer_create_wrap(pixmap.data, pixmap.width,
-			pixmap.height, pixmap.width * 4, true);
+		*buffer = buffer_create_from_data(pixmap.data, pixmap.width,
+			pixmap.height, pixmap.width * 4);
 	}
 }