Use a saner sanity check for floating_reposition.

The function contained_by_output checks whether any output contains any
parts of a give rect. Rather than relying on the central point of the rect.

include/randr.h

@@ -87,6 +87,16 @@ Output *get_output_by_name(const char *name);
  */
 Output *get_output_containing(int x, int y);
 
+/*
+ * In contained_by_output, we check if any active output contains part of the container.
+ * We do this by checking if the output rect is intersected by the Rect.
+ * This is the 2-dimensional counterpart of get_output_containing.
+ * Since we don't actually need the outputs intersected by the given Rect (There could
+ * be many), we just return true or false for convenience.
+ *
+ */
+bool contained_by_output(Rect rect);
+
 /**
  * Gets the output which is the next one in the given direction.
  *