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-png.c

@@ -64,10 +64,20 @@ img_png_load(const char *filename, struct lab_data_buffer **buffer)
 	}
 	cairo_surface_flush(image);
 
-	double w = cairo_image_surface_get_width(image);
-	double h = cairo_image_surface_get_height(image);
-	*buffer = buffer_create_cairo((int)w, (int)h, 1.0, true);
+	if (cairo_image_surface_get_format(image) == CAIRO_FORMAT_ARGB32) {
+		/* we can wrap ARGB32 image surfaces directly */
+		*buffer = buffer_adopt_cairo_surface(image);
+		return;
+	}
+
+	/* convert to ARGB32 by painting to a new surface */
+	uint32_t w = cairo_image_surface_get_width(image);
+	uint32_t h = cairo_image_surface_get_height(image);
+	*buffer = buffer_create_cairo(w, h, 1);
 	cairo_t *cairo = (*buffer)->cairo;
 	cairo_set_source_surface(cairo, image, 0, 0);
-	cairo_paint_with_alpha(cairo, 1.0);
+	cairo_paint(cairo);
+	cairo_surface_flush((*buffer)->surface);
+	/* destroy original surface */
+	cairo_surface_destroy(image);
 }