avcodec/proresenc_kostya: Factor flushing PutBitContext out

avcodec/proresenc_kostya: Factor flushing PutBitContext out

The function to write an ordinary (luma or chroma) plane as well as
the function for writing an alpha plane have some similarities:
They record the initial bitposition (despite said position always being
byte-aligned), flush the PutBitContext themselves and return the amount
of bytes they wrote.

This commit factors this out; it also replaces bitpositions by
bytepositions and it avoids recording the initial byteposition because
said information is already available from the position at the end of
the last plane.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>