Ideogram: Mouse coordinate padding is off at the end of each chromatin fiber row

[josiahseaman]

#39 #53 Follow on for Ideogram 44c664e

This is a known issue that won't be fixed right now.

padding[-1] = self.fibre_padding  # very last y layer has padding to match hacked_padding()
# This has the side effect that coordinates are wrong for the last half of the last
# block in each row where hacked padding comes in the middle and mouse padding is at the end

The solution is to either rewrite build_coordinate_mapping() to be levels based or special case the last block of each fiber row in peano_mouse_position().