ExecuteBuffer for that case if you already know the size of your rendered template.
Change ExecuteWriter to ExecuteBuffer
Change ExecuteWriter to ExecuteBuffer again
Merge remote-tracking branch 'upstream/master'
Merge pull request #63 from fromYukki/master

New API-function: Template.ExecuteBuffer()
ExecuteBuffer() prevents pongo2 to allocate a new buffer for template output generation; instead, the API user can provide his own allocated buffer for performance reasons.