Place float children of floats correctly in the flow tree.
Fix the handling of flow_contexts in floats.

The num_floats was hard-coded to 1 and didn't take into account the
children. Also the float context was not being threaded through the children
properly.
auto merge of #676 : metajack/servo/acid1-all-boxes, r=eric93

This makes acid1 have all 6 boxes show up, and almost correctly positioned.

r? @eric93