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.

cov-core==1.15.0
coverage==4.0.3
invoke==0.12.2
mock==1.0.1
pytest==2.8.7
pytest-cov==2.2.1
pytest-mock==0.10.1