stride value is not relevant with unpadded content and the total count
of pixels (width x height) must be used instead of the rounding based on
width only then multiplied by height

unpadded_10bit value computing is moved sooner in the code in order to
be able to use it during computing of minimal content size. Also make sure to
only set it for 10bit.

Fix 'Overread buffer' error when the content is not lucky enough to have
(enough) padding bytes at the end for not being rejected by the formula
based on the stride value

Fixes ticket #10259.
Signed-off-by: Jerome Martinez <jerome@mediaarea.net>
Signed-off-by: Marton Balint <cus@passwd.hu>