common canvas: revise clear method.

common canvas: revise clear method.

Canvas::clear() introduces a new argument "free" that deterimes freeing the retained paints.

If free is true, Canvas won't delete the retained paints so that user keep paints valid.

In this scenario, user must have paints pointers, free them manually or push them again to the canvas.

This scenario is useful if user wants to re-organize paints order in the list or reuse them.

Change-Id: Ib8c9125dafed301430eee4cd293434b8d84c0ed7