Check variable referencing, overwriting, memory use in Renderers

[david]

Assigned templates, while retrieved and assigned as reference whereever possible, likely aren't always kept as a reference, due to the use of array_merge(). Check that, memory footprint should be as low as possible. Some, like PHPTAL, can't assign as reference, but that's not the issue anyway, since PHP is copy on change, not copy on assign. The crucial point is where the slots are merged back into the variable array inside decorate(). We might have to assign each key by hand to keep all references there, otherwise we end up with a huge copy (not sure, arrays are a bit special in this regard).