Bug 880836. Add MaskSurface() to Azure. r=bas

This gives a substantial performance improvement over the more general Mask():

Test (MaskSurface10x10):  1.93042ms +/- 0.305028
Test (MaskSurface100x100):  2.33435ms +/- 1.00807
Test (MaskSurface500x500):  28.0472ms +/- 0.820744
Test (MaskSurface1000x1000):  107.656ms +/- 6.44615
Test (Mask10x10):  13.5237ms +/- 0.0932034
Test (Mask100x100):  14.0742ms +/- 1.21005
Test (Mask500x500):  96.5596ms +/- 1.81415
Test (Mask1000x1000):  356.891ms +/- 9.30592

--HG--
extra : rebase_source : a3031305302fb4abe93aa480df35ed0936d68cd8