prores: decode alpha plane when it's present
Escape 130 (RPL) decoder

Some fixes provided by Paul B Mahol <onemda@gmail.com>
and Michael Niedermayer <michaelni@gmx.at> and me.

Signed-off-by: Diego Biurrun <diego@biurrun.de>
Signed-off-by: Kostya Shishkov <kostya.shishkov@gmail.com>