Add API for extracting image size from a file

For the asynchronous loading we need a function call that parses
a file, given its path, and retrieves the image width and height.

This commit adds cogl_bitmap_get_size_from_file() to the CoglBitmap
API.

common/cogl-bitmap.c

@@ -149,6 +149,14 @@ _cogl_bitmap_copy_subregion (CoglBitmap *src,
     }
 }
 
+gboolean
+cogl_bitmap_get_size_from_file (const gchar *filename,
+                                gint        *width,
+                                gint        *height)
+{
+  return _cogl_bitmap_get_size_from_file (filename, width, height);
+}
+
 CoglBitmap *
 cogl_bitmap_new_from_file (const gchar    *filename,
                            GError        **error)