This PR fixes a numbe of interconnected bugs:
* We weren't consistently uploading a dirty BitmapData to the render
backend before drawing to/from it.
* BitmapData.draw should *not* add a fill color - it should draw over
the current contents of the BitmapData
* After drawing to a non-transparent BitmapData, we need to manually
set the opacity back to 255 for each pixel (the drawing process
takes transparency into account, but the opacity information is
thrown away at the end).
Aaron Hill