This moves creation of the render task graph to be slightly
earlier during frame building. Instead of creeating the render
tasks during prepare_prims, they are created during the
take_context call for pictures, before recursing into primitives.
This means that in future we can have a surface that targets
multiple render tasks (e.g. for multiple dirty regions), which
may create a different batch set for each region.
Differential Revision: https://phabricator.services.mozilla.com/D29994
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extra : moz-landing-system : lando
effaccc4f6