Allow callers to choose axies for GreedyQuadsBuffer

[malmz]

Add a constructor to GreedyQuadsBuffer to allow for custom Axis3Permutation

[bonsairobo]

I've refactored the greedy quads code a bit to handle this. Now you can pass in any set of QuadGroups you want with GreedyQuadsBuffer::new_with_quad_groups. To get something that works when the +Y axis point UP, use GreedyQuadsBuffer::new_with_y_up.