Exporting still frames via comp.saveFrameAs previously flattened the
rendered frame against the composition background color, stripping
the alpha channel. Single still frames queued in the Render Queue also
defaulted to an RGB-only PNG sequence output module.
Preserve straight alpha by default in comp.saveFrameAs (while honoring
transparent: false if requested), and set TemplateSlot::Still to default
to 'PNG Sequence with Alpha'.
Fixes#1815
Kushagra Kumar
791e6bb5eb