Use renderFlat inside of the RenderFilter

[jasmith-hs]

So that extends roots are not prematurely processed. Similar to how expression nodes use interpreter.renderFlat when doing nested interpretation, the RenderFilter must do the same, otherwise it will process the extends roots early. What this looks like is that the new test I added's output looks like this before this fix:

I am the extension body
You should never see this text in the output!
Base footer

The reason this bug happens is because:

• We encounter the {% extends 'filter/render/base.jinja' %} tag, and add the base.jinja as an extends root
• We encounter the extended {% block body %}
• We use the |render filter
• We then try to process the extends root.
  • We find {% block body %} and {% block footer %} and resolve them to the extended body and the base footer as that is all we've seen at this point
  • We pop the extends root
  • We return the output of base.jinja with the extended body substituted in
  • We encounter the extended {% block footer %}
  • We try to process the extends root, there are none anymore so we act like our original template didn't extend anything and we output it, including its (previously resolved) blocks

What should happen (what now happens after this PR) is:

• We encounter the {% extends 'filter/render/base.jinja' %} tag, and add the base.jinja as an extends root
• We encounter the extended {% block body %}
• We use the |render filter
• We encounter the extended {% block footer %}
• We try to process the extends root
  • We find {% block body %} and {% block footer %} and resolve them to the extended body and the extended footer as we've seen both of those already
  • We pop the extends root
  • We return the output of base.jinja with the extended body and footer substituted in