render: Change how we store render passes info. Previously we used a dictionary with integers as keys, which doesn't go well with JSON serialization. Now replace with an array of fixed length with items that are `None` by default.

# humanstxt.org/
# The humans responsible & technology colophon

# AUTHORS

Ludovic Chabant -- @ludovicchabant

# TECHNOLOGY

PieCrust -- http://bolt80.com/piecrust

# THANKS