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/scaled-scene-buffer.c

@@ -72,8 +72,8 @@ _update_buffer(struct scaled_scene_buffer *self, double scale)
 		/* Ensure the buffer doesn't get deleted behind our back */
 		wlr_buffer_lock(&buffer->base);
 	}
-	self->width = buffer ? buffer->unscaled_width : 0;
-	self->height = buffer ? buffer->unscaled_height : 0;
+	self->width = buffer ? buffer->logical_width : 0;
+	self->height = buffer ? buffer->logical_height : 0;
 
 	/* Create or reuse cache entry */
 	if (wl_list_length(&self->cache) < LAB_SCALED_BUFFER_MAX_CACHE) {