Again, gl/vulkan not working yet (on the assumption that sw would be
trickier).
Fisheye overrides water warp because updating the projection map every
frame is far too expensive.
I've added a post-process pass to the interface in order to hide the
implementation details, but I'm not sure I'm happy about how the
multi-pass rendering for cube maps is handled (or having the frame
buffers as exposed as they are), but mainly because Vulkan will make
implementation interesting.
