screenshooter: correct output ordering of screencast recording when Y-flip is off

screenshooter: correct output ordering of screencast recording when Y-flip is off

The screenshooter encoder wrote the output from either top-to-bottom or
bottom-to-top, depending on the Y-flip setting, but wcap-decode only
decodes from bottom-to-top. Make the encoder always output from
bottom-to-top, to match the decoder, and flip the input (source)
according to the Y-flip setting.

Signed-off-by: Tomohito Esaki <etom@igel.co.jp>
Reviewed-by: Giulio Camuffo <giuliocamuffo@gmail.com>