screen-share: use read_format consistently

This was using read_format for the read_pixels() call, and then using a
hardcoded format for interpreting the data received from read_pixels().
That works only by accident, read_format being the same as the hardcoded
format.

Use read_format for the interpreting too. This should guarantee the read
pixels are processed correctly.

Found by code inspection.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>