certain blend operations cannot be applied to > 2 grobs simultaneously

[mjskay]

Certain blend ops (e.g. "atop") can only be applied to pairs of grobs using grid::groupGrob(src, op, dst) instead of grid::groupGrob(src, op) where src is (say) a grobTree or some other collection of grobs.

Need to fix blend() so that it is compatible with these binary-only ops to allow using ops like "atop". One option is to maintain a table of ops known to be compatible with > 2 grob blending and fall back to a Reduce() for the other ops (since that should definitely work but will be slower).