avcodec/proresenc_kostya: set initial max_slice_size based on frame_size_upper_bound

If the initial max_slice_size is 0 then reallocation is disabled for the first
slice.

Reviewed-by: Christophe Gisquet <christophe.gisquet@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/proresenc_kostya: allocate 1 slice more to avoid triggering the reallocation warning when the used space is actually less than the allocated

Reviewed-by: Christophe Gisquet <christophe.gisquet@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>