'**SUMMARY**
Push a new drawing layer onto the back of the layer stack
**PARAMETERS**
* *layer* - The new drawing layer to add.
**RETURNS**
The index of the new layer as an integer.
**EXAMPLE**
>>> img = Image("Lenna")
>>> myLayer = DrawingLayer((img.width,img.height))
>>> img.addDrawingLayer(myLayer)
**SEE ALSO**
:py:class:`DrawingLayer`
:py:meth:`insertDrawinglayer`
:py:meth:`addDrawinglayer`
:py:meth:`dl`
:py:meth:`toPygameSurface`
:py:meth:`getDrawingLayer`
:py:meth:`removeDrawingLayer`
:py:meth:`clearLayers`
:py:meth:`layers`
:py:meth:`mergedLayers`
:py:meth:`applyLayers`
:py:meth:`drawText`
:py:meth:`drawRectangle`
:py:meth:`drawCircle`
:py:meth:`blit`'

def addDrawingLayer(self, layer=None):

    if (not isinstance(layer, DrawingLayer)):
        return 'Please    pass    a    DrawingLayer    object'
    if (not layer):
        layer = DrawingLayer(self.size())
    self._mLayers.append(layer)
    return (len(self._mLayers) - 1)
