bytestream: add functions for accessing size of buffer

Signed-off-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: Justin Ruggles <justin.ruggles@gmail.com>

CC:libav-stable@libav.org
(cherry picked from commit de9d2705f61ef569487ec5f8974a9c7ce34ec783)
bytestream: add bytestream2_copy_buffer() functions

This is basically an overread/overwrite-safe memcpy between a
GetByteContext and a PutByteContext.

CC:libav-stable@libav.org
(cherry picked from commit 5748faf291fec297ef25d81962b52b3438f54278)