Agg clip rendering fix

This was discovered during the discussion in the e-mail thread "removing paths inside polygon".

When paths are affected by a clip path, the scanline type being used for rendering was wrong.  It was `grey8`, but should be `no_clip_grey8`.  This resulted in horizontal lines looking thicker than they should.

This also fixes a related performance bug where the buffer for the alpha mask was over-allocated by a factor of 4.