Bug 691314

Commit, rev 11213 fixed a problem when using GraphicsAlphaBits in the presence of patterns.  The fix involved intialializing the pattern's bit buffer appropriately so that when gx_default_copy_alpha performed a get_bits command it would obtatin valid data.   It would be better to have the pattern accumulator set up with it's own copy_alpha opereation that made use of the mask buffer in the pattern object that keeps track of data that has been drawn in the pattern's buffer.  The copy_alpha will need to update the mask to ensure that the anti-aliased graphic is completely drawn when the tiling operation occurs as well as make use of the mask to only compute the alpha from those values.